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.

Alon

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'92 300CE - Sold
2004 C240 - 744 - C7 Wheels - Android Radio
2002 C320 - 816 - Sport Wagon - Sold
2006 E350 Wagon - 368 - Dead
2015 GL350 Bluetec - 197U