Android 2.3.6 = Stock, fast from my experience in 2012.
Android 4.1.2 = Stock, Lags and freezing. [because of TouchWiz bloatwares]
Android 4.2.2 = Haven't tried it.
Android 4.3.1 = Haven't tried it.
Android 4.4.4 = Fast and smooth. *Recommended for daily use
Android 5.1.1 = Lags and unusable. *Older builds
Android 6.0.1 = Fast and smooth. *Also good for daily use
Android 7.1.2 = Fast but unusable with Gapps. *Can be use daily but don't
Android 8.1.0 = Fast but stutters on notification bar, recent app, default launcher, etc. *Older builds
Android 8.1 Go = Lags and unusable. *Older builds
Android 9.0.0 = Slow but expected, since it's still in alpha stage.
HDPIGapps working fine.
OpenGapps Setup Wizard and a lot of systems app crash constantly for Android 6, 7 & 8.
Haven't tried MicoG yet.
Custom ROMs that I tested:
Android 4.4.4 = LiquidSmooth Rom (iiD4x), CM11 (TeamCanjica), CM11 (Chrono), CM11 (A2N).
Android 5.1.1 = CM12.1 (Straciatella), OmniROM (Chrono).
Android 6.0.1 = UltimateROM (Chrono), LineageOS 13 (SergeyL).
Android 7.1.2 = Resurrection Remix-N (Chrono), LineageOS 14.1 (SergeyL).
Android 8.1.0 = LineageOS 15.1 (SergeyL), LineageOS 15.1 Go Edition (Chrono).
Android 9.0.0 = LineageOS 16 (Chrono).
-None at the moment-
Last updated: 21 Nov 2018