I had to go back from 10.11.x to 10.10.7 because the performance was bad, but I'm very confident they've fixed all the performance problems with this 12.0 release.

I'm also confident they've found most of the bugs and have produced something you can just upgrade to, given they've had 7 release candidates over the past few months:

   12.0 RC1 - Jun 22
   12.0 RC2 - Jun 28
   12.0 RC3 - Jul 22
   12.0 RC4 - Aug 02
   12.0 RC5 - Aug 11
   12.0 RC6 - Aug 26
   12.0 RC7 - Sep 01
   12.0     - Sep 08

Overall it looks like a lot of solid testing has gone into this, I shouldn't have to go back to 10.10 again.

UPDATE: Upgraded to 12.0, performance is great, better than 10.10 and 10.11, including in third party apps! No problems with the migration.