Watch history is stored as up to ~150 separately NIP-44-encrypted chunk
events. Each chunk built a fresh cipher suite whose conversation-key cache
lived only in its own closure, so the ECDH conversation key (~12ms each in
the bundle) was re-derived once per chunk — multiplied across actor-candidate
and scheme-fallback retries this synchronously blocked the main thread for up
to ~20s after login/profile open (the reported freeze + fan spin-up).
Memoize the derived key by (scheme, privateKey, target) at module scope so it
is computed once per load and reused. Cleared on logout/identity change next
to clearDecryptionSchemeCache so a derived shared secret never outlives its
session. On the 150-chunk path this drops decrypt time ~1955ms -> ~182ms.
Adds tests/nostr/watchHistoryConversationKeyCache.test.js covering: 150 chunks
=> one derivation, distinct identities derive independently (no cross-identity
reuse), and cache-clear forces re-derivation.
Co-Authored-By: Claude Opus 4.8 <[email protected]>
