Mac OS 9 felt pretty darned fast on my 400Mhz iMac G3 back in 2000. Same for Windows 2000 on my parents’ PIII 750Mhz Dimension 4100. The only time anything felt slow is when a significant amount of data needed to be loaded from their hard drives.
Not all machines were like this though, we also had a Compaq Presario with some kind of Celeron running 98SE and that thing did feel slow more often than not, especially after several months of usage with the cruft buildup that comes with that.
I think the rule (at least while Moore's law was in force), was UIs start out boated but become fast as the hardware catches up. For instance, your example:
> Mac OS 9 felt pretty darned fast on my 400Mhz iMac G3 back in 2000.
You were using a UI that (at its core) was built for 1984 machine, with sixteen additional years of hardware performance improvements.
Every once in a while I boot up a Mac from 1989, and Mac OS is definitely not snappy on it.
I think if you want speed, you need to find something built for a system far more constrained than the one you're actually using. The choices the developers made to make the system merely usable under those constraints will make it fast once they're removed.
That makes a lot of sense, and I agree. Perhaps a good baseline to develop against today to produce a similar result on modern hardware would be something like a Core 2 Duo or Core i5-750 and Geforce 9600 GT.
Not all machines were like this though, we also had a Compaq Presario with some kind of Celeron running 98SE and that thing did feel slow more often than not, especially after several months of usage with the cruft buildup that comes with that.