Most people's mental model of CC is that "it's just a TUI" but it should really be closer to "a small game engine". For each frame our pipeline constructs a scene graph with React -> layout elements -> rasterize them to a 2d screen -> diff that against the previous screen -> _finally_ use the diff to generate ANSI sequences to draw. We have a ~16ms frame budget so we have roughly ~5ms to go from the React scene graph to ANSI written. You're right that in theory we shouldn't have to do much work, but in practice that's required optimizations at every step.

For the GC pauses specifically, what mattered is predictable performance. More allocations == more GC == more frames where the VM is locked up seemingly doing nothing. On slower machines we were seeing this be in the order of seconds, not ms and when somebody is typing all they feel is the 1 character that's stuttering. Honestly, I was surprised about this too as GC in JS is often not something that's too impactful.