optimize React app rendering performance

[Grigoryan01]

1. Task: https://github.com/rolling-scopes-school/tasks/blob/master/react/modules/tasks/performance/performance.md

2. Done 15.06.2026 / deadline 16.06.2026

3. Score: 100 / 100

• Phase 1: Initial Profiling (15/15)

  • Initial profiling documented in PERFORMANCE.md (15)

• Phase 2: Apply Optimizations (70/70)

  • useMemo used for computed values (12)
  • useCallback used for event handlers (12)
  • React.memo used to prevent unnecessary component re-renders (12)
  • Proper key props used for all lists and tables (12)
  • Virtualization implemented for large country list (22)

• Phase 3: Final Profiling (15/15)

  • Final profiling documented with comparison to baseline in PERFORMANCE.md (15)

• Additional Features

  • Performance bottlenecks identified using React DevTools Profiler
  • Before/after performance metrics comparison included
  • Commit duration improvements documented
  • Render duration improvements documented
  • Flame chart screenshots provided
  • Percentage improvements calculated and documented

• Penalty Points

  • PERFORMANCE.md included
  • No commits after deadline
  • Pull Request follows RS School guidelines