Lua is a very efficient language, so maybe what happened before SM5 is that the timing windows were being pushed slightly late by slow code, allowing the player to fall slightly behind the music with no penalty (which in my experience is far more common than jumping ahead of the music). This would also have appreciable effects on things such as calibration and sync. Perhaps when SM5 came along, the inefficiencies that affected the timing windows weren't there anymore, and so the timing became more accurate...but they'd definitely
feel all fucked up to players who've been used to the old 3.95 timing, and it'd also mean that song syncs/global offsets/input lag compensation might be off too, furthering the fuckup.
Even if the timing shift didn't happen exactly as I've described it here, it seems that it would have similar consequences. This needs more detailed investigation...maybe we need to build a "tapper machine" to test things, haha.
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What I find most interesting about this thread is the revelation that SM5 is particularly not performing well on certain video cards and such. See
here and
here.
Performance is a legitimate issue and a good reason not to switch, but when SM5 plays nice with one brand of video card and not another, or running flawlessly on cheap and/or old hardware (mine included) while choking on big gaming rigs, it's obvious that the problem runs deeper than just "SM5 is overhyped bloatware lol"