If you were a windows guy, you would have been far less reluctant to blame the OS!
FWIW I run a Mac Mini i7 quad server from mid 2011 with Logic Pro9 and Reaper on it & am about to try Mixbus 4 on the same machine. I am on Mavericks because Logic Pro9 doesnt like anything past Mavericks, but so far I have had no issues with LP9 or Reaper.
Be funny if it turned out to be Apples "upgrades and improvements" that were the sole cause of your problems.
Me? I haven't 100% trusted any OS since I was forced to give up OS9/OSK back in the eighties!