Draw call

A draw call is one instruction from the CPU to the GPU: 'render this chunk of geometry with this material and this state'. Each one carries setup overhead on the CPU side, and the browser adds its own validation on top in WebGL. Pile up enough of them and the CPU becomes the bottleneck, with the GPU idling while it waits to be told what to do. That is not the same as saying the GPU is never the limit: fill rate, shader cost, texture bandwidth, and overdraw put plenty of WebGL scenes firmly on the GPU side, so profile both ends before picking a fix. Rough field guide: a few hundred draw calls per frame is comfortable, a couple thousand starts hurting on mid-range phones.

Count goes up with the number of separate mesh-plus-material pairs, not with triangle count. This is why one 200k-triangle model often outperforms 400 tiny models that total 50k triangles. The fixes are all about doing more per call: instancing for repeated objects (one call for ten thousand trees), merging static meshes that share a material, and packing textures into an atlas so more objects can share one material.

Gotcha: people optimize the wrong number. Before you decimate meshes, open the renderer stats panel and read the actual call count. And watch the sneaky multipliers: every shadow-casting light re-renders the scene into its shadow map, and every post-processing pass adds another full-screen draw over the whole frame (not a second run through your geometry, but real pixels all the same), so 'one' scene can quietly cost several passes.