diff --git a/backend/database.py b/backend/database.py
index 232c503..c1b3f9f 100644
--- a/backend/database.py
+++ b/backend/database.py
@@ -26,6 +26,9 @@ def get_connection() -> sqlite3.Connection:
     conn.execute("PRAGMA journal_mode=WAL")
     conn.execute("PRAGMA foreign_keys=ON")
     conn.execute("PRAGMA busy_timeout=5000")
+    conn.execute("PRAGMA cache_size = -32000")   # 32MB Page-Cache
+    conn.execute("PRAGMA temp_store = MEMORY")   # Temp-Tabellen im RAM statt auf Disk
+    conn.execute("PRAGMA mmap_size = 268435456") # 256MB Memory-Mapped I/O
     conn.create_function('norm', 1, _norm)
     return conn
 
@@ -1212,3 +1215,11 @@ def _migrate(conn_factory):
             )
         """)
         logger.info("Migration: dogs.gewicht_kg aus health synchronisiert.")
+
+        # Performance-Indizes für häufige Queries
+        conn.execute("CREATE INDEX IF NOT EXISTS idx_health_naechstes ON health(naechstes) WHERE naechstes IS NOT NULL")
+        conn.execute("CREATE INDEX IF NOT EXISTS idx_ki_calls_user_date ON ki_daily_calls(user_id, date)")
+        conn.execute("CREATE INDEX IF NOT EXISTS idx_events_user_datum ON events(user_id, datum)")
+        conn.execute("CREATE INDEX IF NOT EXISTS idx_diary_created ON diary(dog_id, created_at DESC)")
+        conn.execute("CREATE INDEX IF NOT EXISTS idx_notes_created ON notes(user_id, created_at DESC)")
+        logger.info("Migration: Performance-Indizes bereit.")
diff --git a/backend/main.py b/backend/main.py
index d9e0d71..2bf6b4a 100644
--- a/backend/main.py
+++ b/backend/main.py
@@ -10,6 +10,7 @@ from fastapi import FastAPI, Request
 from fastapi.staticfiles import StaticFiles
 from fastapi.responses import FileResponse, JSONResponse
 from starlette.middleware.base import BaseHTTPMiddleware
+from fastapi.middleware.gzip import GZipMiddleware
 from contextlib import asynccontextmanager
 
 from database import init_db
@@ -98,6 +99,20 @@ class _CacheControlMiddleware(BaseHTTPMiddleware):
 app.add_middleware(_CacheControlMiddleware)
 
 
+class MediaCacheMiddleware(BaseHTTPMiddleware):
+    """Setzt aggressive Cache-Header für /media/-Requests.
+    UUID-basierte Dateinamen ändern sich nie → immutable caching.
+    """
+    async def dispatch(self, request: Request, call_next):
+        response = await call_next(request)
+        if request.url.path.startswith('/media/'):
+            response.headers['Cache-Control'] = 'public, max-age=31536000, immutable'
+        return response
+
+app.add_middleware(MediaCacheMiddleware)
+app.add_middleware(GZipMiddleware, minimum_size=1000)
+
+
 # ------------------------------------------------------------------
 # API-Router registrieren (werden nach und nach hinzugefügt)
 # ------------------------------------------------------------------
diff --git a/backend/media_utils.py b/backend/media_utils.py
index 10d9ec1..8a8698f 100644
--- a/backend/media_utils.py
+++ b/backend/media_utils.py
@@ -162,7 +162,7 @@ _PREVIEW_MAX  = 800   # längste Seite in Pixeln
 
 
 def generate_preview(data: bytes, ext: str) -> bytes | None:
-    """Erzeugt ein JPEG-Preview (max _PREVIEW_MAX px). Gibt None zurück bei Videos/PDFs/Fehler."""
+    """Erzeugt ein WebP-Preview (max _PREVIEW_MAX px). Gibt None zurück bei Videos/PDFs/Fehler."""
     if ext.lower() not in _PREVIEW_EXTS:
         return None
     try:
@@ -172,7 +172,7 @@ def generate_preview(data: bytes, ext: str) -> bytes | None:
         img = img.convert("RGB")
         img.thumbnail((_PREVIEW_MAX, _PREVIEW_MAX), Image.LANCZOS)
         buf = io.BytesIO()
-        img.save(buf, format="JPEG", quality=72, optimize=True)
+        img.save(buf, format="WEBP", quality=80)
         return buf.getvalue()
     except Exception:
         return None
@@ -185,11 +185,11 @@ def preview_url_from(url: str | None) -> str | None:
         return None
     low = url.lower()
     if any(low.endswith(s) for s in (
-        ".mp4", ".webm", ".mov", ".avi", ".pdf", "_thumb.jpg", "_preview.jpg"
+        ".mp4", ".webm", ".mov", ".avi", ".pdf", "_thumb.jpg", "_preview.jpg", "_preview.webp"
     )):
         return None
     base, _ = os.path.splitext(url)
-    return base + "_preview.jpg"
+    return base + "_preview.webp"
 
 
 def extract_video_thumb(video_path: str) -> str | None:
diff --git a/backend/routes/admin.py b/backend/routes/admin.py
index 46448e1..facd4d2 100644
--- a/backend/routes/admin.py
+++ b/backend/routes/admin.py
@@ -829,7 +829,7 @@ async def generate_media_previews(user=Depends(require_admin)):
             base, ext = os.path.splitext(fname)
             if ext.lower() not in _PREVIEW_EXTS:
                 continue
-            preview_path = os.path.join(folder, base + "_preview.jpg")
+            preview_path = os.path.join(folder, base + "_preview.webp")
             if os.path.exists(preview_path):
                 skipped += 1
                 continue
diff --git a/backend/routes/diary.py b/backend/routes/diary.py
index 6610d81..a3dee2b 100644
--- a/backend/routes/diary.py
+++ b/backend/routes/diary.py
@@ -686,7 +686,7 @@ async def upload_media(dog_id: int, entry_id: int,
     elif media_type == "image":
         preview_bytes = generate_preview(raw_data, ext)
         if preview_bytes:
-            preview_path = os.path.splitext(path)[0] + "_preview.jpg"
+            preview_path = os.path.splitext(path)[0] + "_preview.webp"
             with open(preview_path, "wb") as f:
                 f.write(preview_bytes)
 
diff --git a/backend/routes/forum.py b/backend/routes/forum.py
index 18f8102..d22a460 100644
--- a/backend/routes/forum.py
+++ b/backend/routes/forum.py
@@ -90,7 +90,7 @@ def _save_upload(file: UploadFile, data: bytes) -> str:
     else:
         preview_bytes = generate_preview(data, ext)
         if preview_bytes:
-            with open(os.path.splitext(path)[0] + "_preview.jpg", "wb") as f:
+            with open(os.path.splitext(path)[0] + "_preview.webp", "wb") as f:
                 f.write(preview_bytes)
     return f"/media/forum/{filename}"
 
diff --git a/backend/static/js/app.js b/backend/static/js/app.js
index e6c8974..187591e 100644
--- a/backend/static/js/app.js
+++ b/backend/static/js/app.js
@@ -3,7 +3,7 @@
    Router, State-Management, Navigation, Initialisierung.
    ============================================================ */
 
-const APP_VER = '416'; // ← bei jedem Deploy mit Frontend-Änderungen erhöhen
+const APP_VER = '417'; // ← bei jedem Deploy mit Frontend-Änderungen erhöhen
 
 const App = (() => {
 
diff --git a/backend/static/js/pages/diary.js b/backend/static/js/pages/diary.js
index 076653f..a39eccd 100644
--- a/backend/static/js/pages/diary.js
+++ b/backend/static/js/pages/diary.js
@@ -598,7 +598,9 @@ window.Page_diary = (() => {
     content.innerHTML = `<div class="diary-media-mosaic">${
       allMedia.map(m => `
         <div class="diary-mosaic-item" data-entry-id="${m.entryId}" data-full-url="${UI.escape(m.url)}">
-          <img src="${UI.escape(m.preview_url || m.url)}" alt="" loading="lazy"
+          <img src="${UI.escape(m.preview_url || m.url)}"
+               ${m.preview_url ? `srcset="${UI.escape(m.preview_url)} 800w, ${UI.escape(m.url)} 2000w" sizes="(max-width:400px) 200px, 400px"` : ''}
+               alt="" loading="lazy"
                onerror="this.src='${UI.escape(m.url)}'">
         </div>`).join('')
     }</div>`;
@@ -839,7 +841,10 @@ window.Page_diary = (() => {
         </div>`;
       } else {
         photoHtml = `<div class="diary-card-photo">
-          <img src="${e.cover_preview_url || e.cover_url || coverMedia.preview_url || coverMedia.url}" alt="Foto" loading="lazy">
+          <img src="${e.cover_preview_url || e.cover_url || coverMedia.preview_url || coverMedia.url}"
+               ${(e.cover_preview_url && e.cover_url) ? `srcset="${UI.escape(e.cover_preview_url)} 800w, ${UI.escape(e.cover_url)} 2000w" sizes="(max-width:600px) 300px, 600px"` : ''}
+               alt="Foto" loading="lazy"
+               ${e.cover_url ? `onerror="this.src='${UI.escape(e.cover_url)}'"` : ''}>
           ${mediaCount > 1 ? `<span class="diary-card-media-count">${mediaCount}</span>` : ''}
         </div>`;
       }
diff --git a/backend/static/js/pages/forum.js b/backend/static/js/pages/forum.js
index fd601a5..c46fe6a 100644
--- a/backend/static/js/pages/forum.js
+++ b/backend/static/js/pages/forum.js
@@ -250,7 +250,10 @@ window.Page_forum = (() => {
     const fotoHtml = t.foto_preview
       ? /\.(mp4|mov|webm|m4v|avi)$/i.test(t.foto_preview)
         ? `<div class="forum-card-thumb forum-card-thumb--video" style="display:flex;align-items:center;justify-content:center;background:var(--c-surface-2)">${UI.icon('video-camera')}</div>`
-        : `<img class="forum-card-thumb" src="${_esc(t.foto_preview_url || t.foto_preview)}" alt="" loading="lazy" onerror="this.src='${_esc(t.foto_preview)}'">`
+        : `<img class="forum-card-thumb" src="${_esc(t.foto_preview_url || t.foto_preview)}"
+             ${(t.foto_preview_url && t.foto_preview) ? `srcset="${_esc(t.foto_preview_url)} 800w" sizes="120px"` : ''}
+             alt="" loading="lazy"
+             onerror="this.src='${_esc(t.foto_preview)}'">`
       : '';
 
     return `
diff --git a/backend/static/sw.js b/backend/static/sw.js
index 45fa3fe..125a592 100644
--- a/backend/static/sw.js
+++ b/backend/static/sw.js
@@ -3,7 +3,7 @@
    Offline-Cache + Push Notifications + Tile-Cache
    ============================================================ */
 
-const CACHE_VERSION = 'by-v437';
+const CACHE_VERSION = 'by-v438';
 const CACHE_STATIC  = `${CACHE_VERSION}-static`;
 const CACHE_TILES   = 'ban-yaro-tiles-v1';  // bleibt über SW-Updates erhalten