diff --git a/web/public/sw.js b/web/public/sw.js
index b337a76..d9ac3e3 100644
--- a/web/public/sw.js
+++ b/web/public/sw.js
@@ -4,7 +4,17 @@
 // cached answer for any of those would be a lie.
 
 const SHELL = "musikdelphin-shell-v1";
-const COVERS = "musikdelphin-covers-v1";
+// v2: the backend now downscales cover art to a 640 px long edge on its way into the
+// cache (MAX_COVER_PX in library/cache.py). The URL of a cover does not change when its
+// contents do, and the handler below is cache-first, so every client that had ever
+// loaded a cover went on serving the old 3000 px one from disk - a 9-megapixel decode
+// per card, which a trace on musicdolphin showed costing ~1.5 s each and dwarfing
+// everything else the page did. Renaming the cache is what retires them: `activate`
+// deletes every cache that is not one of these two.
+//
+// So: change how covers are produced, bump this name. "Immutable per album id" is true
+// of which album a cover belongs to, not of the bytes.
+const COVERS = "musikdelphin-covers-v2";
 
 self.addEventListener("install", (event) => {
   event.waitUntil(
@@ -42,7 +52,9 @@ self.addEventListener("fetch", (event) => {
   const url = new URL(request.url);
   if (url.origin !== self.location.origin) return;
 
-  // Cover art is immutable per album id and is the only heavy thing here.
+  // Cover art is cached aggressively - it is the only heavy thing here, and an album's
+  // art does not change from one load to the next. See the note on COVERS above for
+  // what to do when the way it is *produced* changes.
   if (url.pathname.startsWith("/api/albums/")) {
     event.respondWith(
       caches.open(COVERS).then(async (cache) => {