Q: Since I downloaded Mac OS Sierra my computer slowed down. Any reason for that?

try downloading the free program called etrecheck and run it and post it's result that should bring light into what is troubling your performance

Every OS upgrade I suggest, with interesting results:

- download (system's name) Cache Cleaner from Northern Softworks - Sierra Cache Cleaner

- perform all Maintainance routines with exclusion of virus check and Permissions Repair

- reboot

- perform All and Deep Cache Cleaning (this is the mosty important)

- reboot (just the first will be longer)

- perform Disk Permission Repair - although this is not recommended by Apple and Sierra and "past friends" should have and internal automatic permission repair, IMHO, it does his job not completely

- reboot

This assuming you have at least 8 Byte RAM. Sierra is a little bit slow with just 4 gbytes

No solution? Let's look elsewhere

Simon