diff --git a/bincio/edit/server.py b/bincio/edit/server.py
index e9ddb1a..7e7b516 100644
--- a/bincio/edit/server.py
+++ b/bincio/edit/server.py
@@ -9,6 +9,7 @@ from typing import Any
 
 from fastapi import FastAPI, File, HTTPException, Request, UploadFile
 from fastapi.middleware.cors import CORSMiddleware
+from fastapi.middleware.gzip import GZipMiddleware
 from fastapi.responses import HTMLResponse, JSONResponse, RedirectResponse
 
 from bincio.edit.ops import SPORTS, STAT_PANELS, VALID_ACTIVITY_ID
@@ -21,6 +22,7 @@ strava_client_secret: str = ""
 
 app = FastAPI(title="BincioActivity Edit Server", docs_url=None, redoc_url=None)
 
+app.add_middleware(GZipMiddleware, minimum_size=1024)
 # Allow localhost origins only — this server is never meant to be public
 app.add_middleware(
     CORSMiddleware,
diff --git a/bincio/extract/writer.py b/bincio/extract/writer.py
index 55e97c7..370d619 100644
--- a/bincio/extract/writer.py
+++ b/bincio/extract/writer.py
@@ -50,6 +50,10 @@ def write_activity(
     source = _infer_source(activity)
     has_gps = metrics.bbox is not None and privacy not in ("no_gps", "private")
 
+    # Build timeseries once — written to a separate file to keep detail JSON small
+    timeseries = build_timeseries(activity.points, activity.started_at, privacy)
+    tag = activity.source_hash[-8:] if activity.source_hash else "unknown"
+
     # ── detail JSON ──────────────────────────────────────────────────────────
     detail: dict = {
         "bas_version": "1.0",
@@ -80,7 +84,7 @@ def write_activity(
         "best_efforts": metrics.best_efforts,
         "best_climb_m": metrics.best_climb_m,
         "laps": [_serialise_lap(lap) for lap in activity.laps],
-        "timeseries": build_timeseries(activity.points, activity.started_at, privacy),
+        "timeseries_url": f"activities/{activity_id}.timeseries.json" if timeseries else None,
         "source": source,
         "source_file": activity.source_file,
         "source_hash": activity.source_hash,
@@ -92,7 +96,6 @@ def write_activity(
 
     if pending:
         # Write to a unique pending file — no collision possible
-        tag = activity.source_hash[-8:] if activity.source_hash else "unknown"
         json_path = acts_dir / f"{activity_id}.{tag}.pending.json"
     else:
         json_path = acts_dir / f"{activity_id}.json"
@@ -104,9 +107,18 @@ def write_activity(
                 activity_id = f"{activity_id}-{activity.source_hash[-6:]}"
                 json_path = acts_dir / f"{activity_id}.json"
                 detail["id"] = activity_id
+                detail["timeseries_url"] = f"activities/{activity_id}.timeseries.json" if timeseries else None
 
     json_path.write_text(json.dumps(detail, indent=2, ensure_ascii=False))
 
+    # ── timeseries JSON (separate file — lazy-loaded by the UI) ─────────────
+    if timeseries:
+        if pending:
+            ts_path = acts_dir / f"{activity_id}.{tag}.pending.timeseries.json"
+        else:
+            ts_path = acts_dir / f"{activity_id}.timeseries.json"
+        ts_path.write_text(json.dumps(timeseries, indent=2, ensure_ascii=False))
+
     # ── GeoJSON track ────────────────────────────────────────────────────────
     if has_gps:
         geojson = build_geojson(activity.points, activity_id, epsilon=rdp_epsilon)
@@ -150,6 +162,7 @@ def finalize_pending(output_dir: Path, activity_id: str, source_hash: str) -> st
 
     pending_json = acts_dir / f"{activity_id}.{tag}.pending.json"
     pending_geojson = acts_dir / f"{activity_id}.{tag}.pending.geojson"
+    pending_ts = acts_dir / f"{activity_id}.{tag}.pending.timeseries.json"
 
     final_id = activity_id
     final_json = acts_dir / f"{final_id}.json"
@@ -165,6 +178,8 @@ def finalize_pending(output_dir: Path, activity_id: str, source_hash: str) -> st
     if final_id != activity_id and pending_json.exists():
         detail = json.loads(pending_json.read_text(encoding="utf-8"))
         detail["id"] = final_id
+        if detail.get("timeseries_url"):
+            detail["timeseries_url"] = f"activities/{final_id}.timeseries.json"
         pending_json.write_text(json.dumps(detail, indent=2, ensure_ascii=False))
 
     # Atomic rename: pending → final
@@ -180,6 +195,10 @@ def finalize_pending(output_dir: Path, activity_id: str, source_hash: str) -> st
             pending_geojson.write_text(json.dumps(geo, indent=2, ensure_ascii=False))
         pending_geojson.rename(final_geojson)
 
+    final_ts = acts_dir / f"{final_id}.timeseries.json"
+    if pending_ts.exists():
+        pending_ts.rename(final_ts)
+
     return final_id
 
 
@@ -187,7 +206,7 @@ def cleanup_pending(output_dir: Path, activity_id: str, source_hash: str) -> Non
     """Remove pending files for a losing activity (the one not chosen as canonical)."""
     acts_dir = output_dir / "activities"
     tag = source_hash[-8:] if source_hash else "unknown"
-    for suffix in (".pending.json", ".pending.geojson"):
+    for suffix in (".pending.json", ".pending.geojson", ".pending.timeseries.json"):
         p = acts_dir / f"{activity_id}.{tag}{suffix}"
         p.unlink(missing_ok=True)
 
diff --git a/bincio/render/merge.py b/bincio/render/merge.py
index fb8bfb6..8064776 100644
--- a/bincio/render/merge.py
+++ b/bincio/render/merge.py
@@ -109,6 +109,14 @@ def merge_one(data_dir: Path, activity_id: str) -> None:
 
     needs_merge = has_sidecar or bool(image_files)
 
+    # Symlink the timeseries file (never merged — always points to the extract output)
+    ts_src = acts_dir / f"{activity_id}.timeseries.json"
+    ts_dest = merged_acts / f"{activity_id}.timeseries.json"
+    if ts_dest.exists() or ts_dest.is_symlink():
+        ts_dest.unlink()
+    if ts_src.exists():
+        ts_dest.symlink_to(ts_src.resolve())
+
     # Remove the old dest (symlink or file) before writing the new one
     if dest.exists() or dest.is_symlink():
         dest.unlink()
diff --git a/bincio/serve/server.py b/bincio/serve/server.py
index 58f3bc1..3b992a4 100644
--- a/bincio/serve/server.py
+++ b/bincio/serve/server.py
@@ -17,6 +17,7 @@ from typing import Any, Optional
 
 from fastapi import Cookie, FastAPI, File, HTTPException, Request, Response, UploadFile
 from fastapi.middleware.cors import CORSMiddleware
+from fastapi.middleware.gzip import GZipMiddleware
 from fastapi.responses import JSONResponse
 
 from bincio.serve.db import (
@@ -61,6 +62,7 @@ def _get_data_dir() -> Path:
 
 app = FastAPI(title="BincioActivity Serve", docs_url=None, redoc_url=None)
 
+app.add_middleware(GZipMiddleware, minimum_size=1024)
 app.add_middleware(
     CORSMiddleware,
     allow_origin_regex=r"https?://localhost(:\d+)?",
diff --git a/site/src/components/ActivityDetail.svelte b/site/src/components/ActivityDetail.svelte
index 5025455..18bae3d 100644
--- a/site/src/components/ActivityDetail.svelte
+++ b/site/src/components/ActivityDetail.svelte
@@ -2,12 +2,12 @@
   import { onMount } from 'svelte';
   import { marked } from 'marked';
   import DOMPurify from 'dompurify';
-  import type { ActivitySummary, ActivityDetail, AthleteZones } from '../lib/types';
+  import type { ActivitySummary, ActivityDetail, AthleteZones, Timeseries } from '../lib/types';
   import { formatDistance, formatDuration, formatElevation, formatSpeed, formatDate, formatTime, sportIcon, sportLabel, sportColor } from '../lib/format';
   import ActivityMap from './ActivityMap.svelte';
   import ActivityCharts from './ActivityCharts.svelte';
   import EditDrawer from './EditDrawer.svelte';
-  import { loadActivity } from '../lib/dataloader';
+  import { loadActivity, loadTimeseries } from '../lib/dataloader';
 
   export let activity: ActivitySummary;
   export let base: string = '/';
@@ -17,6 +17,8 @@
   const editEnabled = editUrl !== '' || import.meta.env.PUBLIC_EDIT_ENABLED === 'true';
 
   let detail: ActivityDetail | null = null;
+  let timeseries: Timeseries | null = null;
+  let timeseriesLoading = false;
   let error = '';
   let hoveredIdx: number | null = null;
   let editOpen = false;
@@ -31,8 +33,17 @@
     try {
       detail = await loadActivity(activity.id, activity.detail_url ?? '', base);
       if (!detail) throw new Error('Activity not found');
+      // Use embedded timeseries (IDB activities) or lazy-fetch from URL
+      if (detail.timeseries) {
+        timeseries = detail.timeseries;
+      } else if (detail.timeseries_url) {
+        timeseriesLoading = true;
+        timeseries = await loadTimeseries(detail.timeseries_url, activity.detail_url ?? '', base);
+        timeseriesLoading = false;
+      }
     } catch (e: any) {
       error = e.message;
+      timeseriesLoading = false;
     }
   });
 
@@ -231,7 +242,7 @@
     {#if trackUrl}
       <ActivityMap
         {trackUrl}
-        timeseries={detail?.timeseries ?? null}
+        {timeseries}
         bbox={detail?.bbox ?? null}
         accentColor={color}
         bind:hoveredIdx
@@ -263,11 +274,11 @@
 <!-- Charts -->
 {#if error}
   <p class="text-red-400 text-sm mt-4">{error}</p>
-{:else if detail?.timeseries && detail.timeseries.t.length > 0}
+{:else if timeseries && timeseries.t.length > 0}
   <div class="bg-zinc-900 rounded-xl border border-zinc-800 p-4">
-    <ActivityCharts timeseries={detail.timeseries} bind:hoveredIdx {athlete} />
+    <ActivityCharts {timeseries} bind:hoveredIdx {athlete} />
   </div>
-{:else if !detail}
+{:else if !detail || timeseriesLoading}
   <div class="bg-zinc-900 rounded-xl border border-zinc-800 p-4 h-32 animate-pulse"></div>
 {/if}
 
diff --git a/site/src/lib/dataloader.ts b/site/src/lib/dataloader.ts
index 3933a59..45a998e 100644
--- a/site/src/lib/dataloader.ts
+++ b/site/src/lib/dataloader.ts
@@ -15,7 +15,7 @@
  *   for anything the user recorded or converted on this device).
  */
 
-import type { ActivityDetail, ActivitySummary, BASIndex } from './types';
+import type { ActivityDetail, ActivitySummary, BASIndex, Timeseries } from './types';
 import { listLocalActivities } from './localstore';
 
 // ── Helpers ───────────────────────────────────────────────────────────────────
@@ -170,6 +170,40 @@ export async function loadActivity(
   }
 }
 
+/**
+ * Fetch the timeseries for an activity. Called lazily when the charts section
+ * is shown, so the initial detail load stays small (~1 KB instead of ~600 KB).
+ *
+ * @param timeseriesUrl Relative path from the detail JSON (e.g. "activities/id.timeseries.json")
+ * @param detailUrl     The URL from which the detail JSON was fetched — used to resolve
+ *                      relative paths correctly in both single- and multi-user modes.
+ * @param baseUrl       Site base URL — fallback when detailUrl is empty
+ */
+export async function loadTimeseries(
+  timeseriesUrl: string,
+  detailUrl: string,
+  baseUrl: string,
+): Promise<Timeseries | null> {
+  try {
+    let url: string;
+    if (timeseriesUrl.startsWith('http')) {
+      url = timeseriesUrl;
+    } else if (detailUrl.startsWith('http')) {
+      // detailUrl is absolute — resolve timeseries relative to its directory
+      const dir = detailUrl.substring(0, detailUrl.lastIndexOf('/') + 1);
+      // timeseriesUrl is "activities/id.timeseries.json" — strip leading "activities/"
+      // because dir already ends with "activities/"
+      const filename = timeseriesUrl.replace(/^activities\//, '');
+      url = `${dir}${filename}`;
+    } else {
+      url = `${baseUrl}data/${timeseriesUrl}`;
+    }
+    return await fetchJSON<Timeseries>(url);
+  } catch {
+    return null;
+  }
+}
+
 /**
  * Load athlete profile. Athlete data is not stored locally yet, so this is
  * always a network fetch with a graceful null on failure.
diff --git a/site/src/lib/types.ts b/site/src/lib/types.ts
index 7c5293b..774377f 100644
--- a/site/src/lib/types.ts
+++ b/site/src/lib/types.ts
@@ -114,7 +114,10 @@ export interface ActivityDetail extends Omit<ActivitySummary, 'detail_url' | 'tr
   start_latlng: [number, number] | null;
   end_latlng: [number, number] | null;
   laps: Lap[];
-  timeseries: Timeseries;
+  /** Embedded timeseries — present for IDB-stored (locally converted) activities. */
+  timeseries?: Timeseries | null;
+  /** URL to fetch the timeseries — present for server-extracted activities. */
+  timeseries_url?: string | null;
   mmp: MmpCurve | null;
   strava_id: string | null;
   duplicate_of: string | null;