Yes, I was going to suggest this also, and if the live session does not show the slowdown problem, you will want to try a full reinstall. Of course, there is always the possibility that the full reinstall works well at first, but later runs into problems after MATE does all it’s updates. If this happens, try again and go step-by-step by doing another full install and NOT selecting the “do updates during install” option. That way, after you make sure that the base install is working, you can then use the desktop ‘Software Updater’ to apply the updates singly or in groups, until you find whatever it is that is causing the slowdown.
Also, since other folks are running similar motherboard hardware and not seeing this slowdown, it pays to not overlook the obvious:
like first unplugging all possibly conflicting hardware.
and that, even with good quality compatible hardware, sometimes simple things like just plugging a Bluetooth or WiFi dongle into a USB3 port directly on your motherboard can make the device almost unusable due to USB3 to 2.4 GHz band interference (if this is an issue with your hardware you can often get around this interference by moving the device to a USB2 port, or by using a short USB extension cable, to relocate the device a short distance away)
or the fact that your WiFi signal can suddenly degrade, apparently for no reason at all, simply because a neighbor installed a new WiFi router which is operating on a channel or in a mode that is causing interference to your signal (often this can be fixed by logging into your router’s web interface, doing a channel scan, and manually overriding your router’s WiFi channel to one more widely separated from other strong nearby signals).
So, with luck, it will hopefully be something simple, but quite honestly, Ubuntu hasn’t been nearly the paragon of stability that it once was, so I would be prepared to do a little troubleshooting - but stick with it, and I think you will find that once you get MATE up and running smoothly, it was well worth the effort.