BeOS was way, way snappier to use on the same hardware than Linux (or Windows) no matter how much you trimmed down your (GUI) Linux.
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.