It wasn't until I joined this forum that I learned a clogged catalytic converter can cause poor performance, and it isn't clear to me why they clog, but some have reported this condition. So actually, the Prep H might be close to the truth. I don't know how you test for a clogged catalytic converter. Best to make sure the timing belt is good, and then that's it's right.

I don't know how you test for a clogged catalytic converter.
You might drive it to a Catalytic Church and see if a Confessional helps.