I played the Megaman series on NES, I think it was Megaman 3(?) that pissed me off so bad that I vowed I'd never touch the crap again. I may have also jumped up and down on the cartridge, I don't recall.
But yes, SNES was definitely the golden console for platformers before or since. I have a vivid memory of playing Star Wars on it and finishing it in one go while my big brother fed me a new brand of crisps that had just come out, "Hjärtan".
And it's both strange and not strange about those slowdowns. But I guess there was simply something in the hardware that couldn't quite cope with fast scrolling, my guess is the CPU, and / or sloppy programming!!