I've been having the exact same issues, computer slowing down to a crawl (usually followed by a bluescreen)
I updated the firmware last night.
However, this morning I woke up to a bluescreen with only two lines on it
"STOP: C0000135 The program can't start because %hs is missing from your computer. Try reinstalling the program to fix this problem."
I also received a STOP F4 code only 30mins ago, when I plugged in an external hdd.