imac system taking up huge block of storage (300 gigs!)

was having performance problems, so i checked my storage in "about this mac"

Apparently, my "system" is taking up 300 GIG of storage (out of 500 gig) on my imac retina 5k 27in late 2015 model running mojave.

I've never had this issue before except when my HD was full to the brim with photos and files. . saving in photoshop is taking forever, I'm crashing and stuff just acts wonky and slow to react.

my actual files / docs, pics and apps are taking up less than 100 gig combined.

what the ****?

any ideas how to purge this problem?

I tried the "time machine snapshot purge" as outlined in someone elses question here in forum. it was of no help.

Thanks!