Looking back those machines weren't that slow
Sure at our school we had a Xenix system with around 30 terminals on a 386 which did get slow when 30 pupils were hacking away in Works for Xenix, but then again many modern applications are just as slow on modern hardware. I have seen KMail not keeping up with my typing speed. I have seen computers taking minutes to load the work processing application.
Maybe the reason for this is just bloat as suggested by this talk here:
https://www.youtube.com/watch?v=Nbv9L-WIu0s