Yes a plugged cat can be a cause of slowness.
My bro's 85 190e had a clogged cat, in neutral it would rev fine, in gear you put your foot down and car would go maybe 10-20 mph.
After that was fixed, car ran fine.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.