My iMac 2T running Mojave is REALLY slow ==

Thank you = was hoping you'd see something I wasn't - right now I'm going to first reinstall the OS and see if that helps == also deleted some apps that I didn't need and which I figured I could take off == if the reinstall doesn't work I guess I'll bite the bullet and erase and reinstall == unless you think I probably should just go do that because a simple reinstall won't fix the problem. I hadn't thought of the possibility I upgraded some app and now it's triggering that message == will have to sort that out too. Much appreciated.

Well I did erase - but the drive is a SSD drive [Solid State PCI-express Drive] and I had to reformat it to an APFS drive. Now the following 'warning' pops up "Incompatible Disk: This disk uses features that are not supported on this version of Mac OS". The drive is still just as set up on Mojave == this message did not appear before when I had the original Mojave OS on it - I had a fusion drive in the machine but had to replace the drive and we did an SSD drive == but that was last year and the warning which pops up [I can close it] just started recently = is there a way to turn the warning off? I don't want to bring back the old contents [most not all] of my drive from TimeMachine or elsewhere if this message is going to keep popping up.