Performance

How the engine renders

One render loop. All changes mark a layer dirty; one requestAnimationFrame redraws what is dirty.
Two layers. The scene canvas holds the grid, series, axes and drawings. The top canvas holds the crosshair and hover state, so moving the pointer never redraws the scene.
Only visible bars. The visible slice is found by binary search; series iterate start..end only.
Per-pixel aggregation. When several points share a pixel, line and area series keep only the first, lowest, highest and last point of each pixel column, and candles, OHLC and bars draw one aggregated bar per column. The picture is the same as drawing every point, but the cost depends on the chart width, not on the data size.
Level-of-detail index. Large series (10k+ points, LOD_MIN_LENGTH) get a one-time index of 32-point blocks (LOD_BLOCK: min, max, first, last), cached per data array. Zoomed far out, drawing and autoscale read blocks instead of points, so 200k points pan at 60 fps.
Batched paths. Built-in series batch all shapes of one color into a single Path2D (two fills for a whole candlestick chart).
No React per frame. Pan, zoom and crosshair moves don't re-render React components. Hooks like useChartState re-render only when their selected value changes.
Device pixel ratio aware. Canvases are sized in physical pixels and lines are snapped for crisp 1px strokes.

Keep data referentially stable: memoize derived arrays (useMemo) so the engine doesn't re-map x values.
Memoize custom themes (useMemo(() => createTheme(...), [])).
In custom renderables, iterate only scope.start..scope.end, avoid allocating per bar, and use the "top" layer for anything that follows the pointer.
Custom series can reuse the same machinery from @reincharts/series: linePoints (per-pixel aggregation), getLod / lodExtent (block index and exact range min / max), drawOhlcColumns / ohlcExtent (OHLC data) and PathBatch (one path per color). useSeries uses the block index for autoscale automatically.
For datasets in the millions, also pre-aggregate on the server and load detail on zoom.