Because like other commenters that chimed in, I can notice most SPAs, and all SPAs I've used had annoying performance problems. By problems I don't mean a sudden request that takes half a second, but that every UI interaction feels subtly slow.
I don't know where people get these performance targets of "below 100ms and users won't notice". I notice. I notice if UI responses take longer than two-three animation frames, the same way I notice when a game is running at 20 FPS and not 60 FPS.
Fastmail is laggy in places, but mostly OK. But I'm surprised at you choosing Google Office as an example. Google's office suite is a poster child of slow, clunky SPAs.
Because like other commenters that chimed in, I can notice most SPAs, and all SPAs I've used had annoying performance problems. By problems I don't mean a sudden request that takes half a second, but that every UI interaction feels subtly slow.
I don't know where people get these performance targets of "below 100ms and users won't notice". I notice. I notice if UI responses take longer than two-three animation frames, the same way I notice when a game is running at 20 FPS and not 60 FPS.