Here is my story. Few weeks ago I've bought a brand new MacBook Pro 13' (2015 edition). I've opened it and installed macOS Sierra for the first time. This laptop has never been on some other OS. However, I experience some UI lags which are just killing me and my experience. I've never expected this from a new MacBook Pro. For example when I scroll through Facebook or trying to resize some application (even if I try to resize the activity monitor) there is a noticeable lag and 'decrease of the FPS'. Not really expected this kind of issues.
Just for my peace of mind, I've run a few benchmarks (CPU, GPU, SSD), downloaded a few 'normal-weight' games and everything looks OK. Games have I would say normal FPS and when I compare the benchmarks results they are pretty good and almost the same compared to other 2015 MacBook Pro's.
I was told that this UI lag is because of the OS and if I downgrade and wait until the next macOS release, this will be probably fixed. So, OK I want to downgrade, but I can't figure out how.