docs: add comprehensive performance analysis and optimization guide

- Add PERFORMANCE.md with detailed analysis of current performance characteristics
- Document query parsing, bulk operations, connection management, memory usage
- Identify bottlenecks with severity ratings (High/Medium/Low priority)
- Provide actionable optimization opportunities categorized by effort/impact
- Include benchmarking guide with code examples and recommended tools
- Document best practices for production deployments
- Add specific recommendations for:
  - Content-based query caching (50-90% potential hit rate improvement)
  - Bulk operation optimization (reduce round-trips)
  - Elasticsearch bulk helpers integration
  - Streaming API for large datasets
  - Connection pool configuration
  - Memory optimization strategies
- Include working code examples for all optimizations
- Provide complete benchmark suite setup

Author: marshallswain