Because the only update occurring is the changing of text in an input. Yet due to how React works it’s diffing a much larger tree.
Are you sure there isn’t a slow server request happening before the UI update? I would be very surprised that you’re regularly encountering web sites with noticeably slow React reconciliation. It’s difficult to make that slow.
The diffing is very fast. It would take a ridiculously large tree to start running up against the limits of v8.