perf: improve LCP by parallelising board load and caching decrypts#216
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The card body was the dominant LCP element. Three changes shorten the critical path to its first paint: - Board.svelte onMount now starts getRanks and the three firestore subscription handshakes in parallel instead of serialising them, and drops the 200ms fade-in on the cards container. busy=false now flips as soon as ranks REST + the password check resolve, rather than waiting for every subscription to register. - firestore.signIn() is memoised so the parallel subscribers share a single /auth + signInWithCustomToken round-trip. - encryption.decrypt() caches results (LRU, bounded at 1024 entries) so re-renders of the same ciphertext don't re-run PBKDF2's 200k iterations.
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Summary
The Core Web Vitals report flags the card body (
div.p-1.w-100.pre-wrapinCard.svelte) as the dominant LCP element. Three changes shorten the critical path to its first paint:Board.svelteonMount. Previously it serialised six awaits —getBoard→getRanks→checkPassword→ threesubscribeToXcalls (each with its ownsignInround-trip) — before flippingbusy = false. NowgetRanksand the three firestore subscription handshakes run concurrently, andbusy = falseflips as soon as ranks REST + password check resolve. Subscriptions complete in the background; cards/ranks populate as their first snapshots arrive.transition:fadeon the cards container. Pure win — that 200ms was being added on top of the LCP every load.signIn()infirestore.jsso the three concurrent subscribers share one/auth+signInWithCustomTokenround-trip instead of racing three.decrypt()results inencryption.js(LRU, bounded at 1024 entries) so re-renders of the same ciphertext don't redo PBKDF2's 200k iterations. Helps encrypted boards on every reactive update after the first paint.Two further ideas from the analysis were deferred as too invasive for this PR: pre-decrypting cards eagerly into the store, and moving PBKDF2 off the main thread via Web Workers.
Test plan
npm run lintnpm run buildnpx cypress run— full local suite (86 tests across 19 specs) passes, includingEncryption.cy.js,ConnectionLost.cy.js,OwnerRealtimeSync.cy.js,OpenPermission.cy.jshttps://claude.ai/code/session_01CbPv23BpW2H57tvRh7fxqQ
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