I'd happily pay for a new version of Windows XP/7, Office, Adobe CC, ______ that was 20% faster/efficient, with the exact same feature set.
Sadly, I don't think the marketing departments realize this. They honestly believe that only new features sell computers.
You'd pay for speed-ups that come from reworking algorithms to make them more efficient, but how do you avoid someone adding wait loops in strategic places and then selling you the software again after taking them out? IBM sells you an increase in processing power for your mainframe by enabling processors that were already there the whole time but disabled by configuration. I don't think I like that business model.
The key is, I have a baseline now - a useable system that is X fast. OS's are mature products, with several alternatives. That keeps people honest