Had this G5-5090 gaming desktop for a year.  Write performance on the system disk started out at 1,216 MB/s when the system was new.  Now it's closer to 7.5 MB/s.  Yes, that is almost two orders of magnitude less.  Sustained writes (as when updating a big game via Steam) are a particular problem.  Please explain what the I am doing in the situation & how to fix it.

I am willing to buy a new system disk but the frickin' motherboard doesn't come with an extra expansion slot?