PRD: Performance Optimization and Caching for Sliding Sync v4 Endpoint

PRD: Performance Optimization and Caching for Sliding Sync v4 Endpoint

Problem: The new v4 simplified sliding sync endpoint needs performance optimization and caching to support 500+ concurrent users with low latency.

Solution: Implement layered optimization approach: (1) baseline optimizations from existing sync endpoint, (2) v4-specific enhancements, (3) strategies from Synapse implementation, (4) comprehensive caching layer with tunable memory limits.

Detailed PRD: prds/3-performance-optimization-caching.md

Priority: High