Add optimization summary and finalize performance improvements

Co-authored-by: alexlib <[email protected]>

Author: Copilot

OPTIMIZATION_SUMMARY.md

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+# Performance Optimization Summary
+
+## Quick Summary
+
+This PR implements performance optimizations across the OpenPIV codebase to reduce execution time and memory usage.
+
+## Files Changed
+
+- `openpiv/pyprocess.py` - Vectorized array operations, reduced copies
+- `openpiv/validation.py` - Eliminated unnecessary masked array copies
+- `openpiv/filters.py` - Conditional masked array creation
+- `openpiv/test/test_performance.py` - New performance validation tests (NEW)
+- `PERFORMANCE_IMPROVEMENTS.md` - Detailed documentation (NEW)
+
+## Key Optimizations
+
+1. **Vectorized Operations**: Replaced Python loops and list comprehensions with NumPy operations
+2. **Reduced Array Copies**: Eliminated unnecessary copy operations, especially with masked arrays
+3. **Conditional Conversions**: Only convert dtypes when necessary
+4. **Optimized Border Checking**: Use np.maximum/np.minimum instead of array indexing
+
+## Performance Gains
+
+- `find_all_first_peaks`: Fully vectorized, < 10ms for 100 windows
+- `normalize_intensity`: Conditional conversion, < 50ms for 50 windows  
+- `global_std`: No copies for non-masked input, < 10ms for 100x100 arrays
+- `replace_outliers`: Conditional masking, < 100ms for 50x50 arrays
+
+## Testing
+
+✅ All 198 existing tests pass
+✅ 5 new performance tests added
+✅ Total: 203 tests pass in ~8 seconds
+✅ Tutorial scripts verified working
+
+## Backward Compatibility
+
+✅ 100% backward compatible
+- Function signatures unchanged
+- Return types unchanged  
+- Numerical results unchanged
+
+## Documentation
+
+See `PERFORMANCE_IMPROVEMENTS.md` for:
+- Detailed before/after code comparisons
+- Performance metrics
+- Future optimization opportunities
+- General optimization principles
+
+## Impact
+
+These optimizations particularly benefit:
+- Large PIV analysis with many interrogation windows
+- Iterative refinement algorithms
+- High-resolution image processing
+- Batch processing workflows