Alright, one mystery is now solved while bringing up a new one.
I tried Rastan Saga II, Volfied and Space Harrier with my flash card and they all run smooth. I tried another game on the card and it was still choppy. So Chris's artifact reduction theory was correct.
However, as I said earlier, Bonk's Adventure was all choppy. So I loaded a Bonk's Adventure rom and a PC Genjin rom onto the card and tried them out. They both worked fine.
So finally, I tried my real actual official Bonk's Adventure HuCard again.... and it's still choppy. Plus, when I tried my real actual Out Run HuCard, it also looked choppy(
but I couldn't be completely sure).
So either the flash card combined with artifact reduction turned off somehow lets my TV run the games fine, or there are multiple versions of Bonk's Adventure out there, some of which got the artifact reduction turned back on.