UI responsiveness you are according to has nothing to kernel. With Wayland Linux should become very responsive. I'm using Kubuntu and in not UI related tasks it's much more responsive than Windows XP. Those Haiku guys are simply lying or have no clue. I heard Haiku is cheating (preloading itself into memory or something like that) and that's why it seems to be fast. I recommend to try Puppy Linux which kills Haiku in every term.