Downgrade macOS Sierra of a brand new MacBook 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.
Macbook Pro 2015 came with El Captain version pre-installed in factory therefor you can easily revert back to Factory installed OS by using Internet Recovery.
Don't forget to Backup everything. (since it is a new Macbook pro, maybe you don't have much in it yet to backup? otherwise backup everything. Use TimeMachine to backup)

Now, turn off your Macbook. Turn it on and hold Command+Option+R
You will see a rotating earth icon. Connect to your WiFi and let it download and install El Captain from Apple servers.