Updating a cell in Excel is a CPU round trip, whereas updating a cell in Sheets is a network round trip. An operation that touches as few as 1000 lines (e.g. autofill a large range, change a value that feeds into many cells) incurs many seconds of latency as it's fed through the cloud and the new cell data is refreshed. The more rows you have, the slower the response time.

Change cells in parallel, not in sequence.