I have a 6s that's still on 10. Reading this and other comments make me think that maybe 12 won't be too bad. There's a few apps out there that I use that don't work with 10 anymore and so this may be the time for me to upgrade.
I'm pretty curious about how 12 compares to 10 as well. Seems like 11 was definitely worse on performance, but might be too early to compare 12 to earlier versions.