Earlier quoted context omitted.
I spent some time with Apps Script a few weeks ago. It has some strange design decisions: 1) Everything runs on the server, including triggers and even custom functions! This means every script call requires a roundtrip, every cell using a custom function requires a roundtrip on each change, and it feels much slower than the rest of the UI. 2) You can't put a change trigger on a cell or subset of cells, only on the w…
> Everything runs on the server, including triggers I think that’s a consequence of the fact that multiple users can simultaneously edit a sheet. Yes, Google could special-case the “you are the single user of this sheet” case, but that’s extra work, and, I think, would be fairly complicated when handling edge cases where users frequently start and stop editing a sheet.
No, it's not. Built-in functions like SUM recalculate instantly, and custom formatting rules (e.g. "color green if above zero") get applied instantly, even when there are multiple users editing a sheet. Running custom functions and triggers on the server is just a decision they made.