Reactive Dashboard Performance Agent Skill

Reactive Dashboard Performance

Expert in building production-grade reactive dashboards that load in <100ms and have comprehensive test coverage.

Core Expertise

Performance Patterns (Linear, Vercel, Notion-grade)

• Skeleton-First Loading
• Render skeleton immediately (0ms perceived load)
• Stream in data progressively
• Never show spinners for <200ms loads

• Aggressive Caching
• React Query with staleTime: 5min, cacheTime: 30min
• Optimistic updates for mutations
• Prefetch on hover/mount

• Code Splitting
• Route-based splitting (Next.js automatic)
• Component-level lazy() for heavy widgets
• Preload critical paths

• Memoization Strategy
• useMemo for expensive computations
• React.memo for pure components
• useCallback for stable references

Testing Reactive Dashboards

• Mock Strategy
• Mock at service boundary (React Query, analytics)
• Never mock UI components (test real DOM)
• Use MSW for API mocking when possible

• Async Handling

   // WRONG - races with React
   render(<Dashboard />);
   const element = screen.getByText('Welcome');

   // RIGHT - waits for async resolution
   render(<Dashboard />);
   const element = await screen.findByText('Welcome');

• Timeout Debugging
• Timeouts mean: missing mock, wrong query, or component not rendering
• Use screen.debug() to see actual DOM
• Check console for unmocked errors

• Test Wrapper Pattern

   const TestProviders = ({ children }) => (
     <QueryClientProvider client={testQueryClient}>
       <AuthProvider>
         {children}
       </AuthProvider>
     </QueryClientProvider>
   );

Real-World Examples

• Linear Dashboard: Skeleton → Stale data → Fresh data (perceived <50ms)
• Vercel Dashboard: Prefetch on nav hover, optimistic deploys
• Notion Pages: Infinite cache, local-first, sync in background

Diagnostic Protocol

Integration Test Timeouts

• Check what's actually rendering

   render(<Component />);
   screen.debug(); // See actual DOM

• Find unmocked dependencies
• Check console for "not a function" errors
• Look for network requests in test output
• Verify all contexts are provided

• Fix async queries
• Use findBy* instead of getBy*
• Increase timeout if needed: waitFor(() => {...}, { timeout: 3000 })
• Mock React Query properly

• Simplify component tree
• Test widgets individually first
• Add full integration tests last
• Use data-testid for complex queries

Performance Optimization

Dashboard Load Budget

| Phase | Target |

|-------|--------|

| Skeleton render | 0-16ms (1 frame) |

| First data paint | <100ms |

| Full interactive | <200ms |

| Lazy widgets | <500ms |

React Query Config

const queryClient = new QueryClient({
  defaultOptions: {
    queries: {
      staleTime: 5 * 60 * 1000, // 5min
      cacheTime: 30 * 60 * 1000, // 30min
      refetchOnWindowFocus: false,
      refetchOnMount: false,
      retry: 1,
    },
  },
});

Skeleton Pattern

function Dashboard() {
  const { data, isLoading } = useQuery('dashboard', fetchDashboard);

  // Show skeleton immediately, no loading check
  return (
    <div>
      {data ? <RealWidget data={data} /> : <SkeletonWidget />}
    </div>
  );
}

Common Pitfalls

• Spinners for fast loads - Use skeletons instead
• Unmemoized expensive computations - Wrap in useMemo
• Testing implementation details - Test user behavior
• Mocking too much - Mock at boundaries only
• Synchronous test expectations - Everything is async

When debugging test timeouts, ALWAYS start with screen.debug() to see what actually rendered.