I recently upgraded my Macbook Pro to macOS High Sierra and experienced horrible lag and slowness issues after upgrading.
The Macbook was upgraded with an Aura Pro X SSD drive from Macsales. I decided I wanted to to a fresh and clean install of the OS, so I installed from scratch. I would normally expect this to result in a snappy, like-new machine but I noticed horrendous performance after fresh installing OSX.
The fix was to reset the PRAM and SMC on my Macbook. Once I did this after upgrading, the performance has been stellar! Here is how to reset your PRAM and SMC: https://thenextweb.com/lifehacks/2017/06/14/how-when-why-to-reset-the-pram-smc-on-your-mac/