IDK what scheduler voodoo they were doing, but it was awesome.
Only things I've seen that achieved something similar were QNX/Photon, and (though with the benefit of way stronger hardware and a ton of "cheating" by suspending applications) some (mostly early) versions of iOS.
I'm not sure I have any use for Haiku today, but I definitely wish for a world in which computer GUIs didn't feel so damn slow and janky and pre-occupied with whatever it's got going on internally rather than what I need it to be doing right now.
Also, I wish some kind of tagging system for filesystems had taken off well enough that I could rely on it, even cross-platform and when copying files between filesystems. Entire programs could just be file tags. Other programs could just be a thin GUI over tagged files. It sucks that didn't end up becoming a standard and reasonably cross-platform-compatible thing.