I'm pretty sure these slowdowns were on original hardware. I know once I thought I noticed emulator slowdown in SMW but when watching a lets play on original hardware... the slowdown was there as well.
I've read in Spanish forums that there are slowdowns and frame drops in some games that weren't present in their original versions.
That's because often the games over here in Europe wouldn't slow down in the same place that the US version's did, due to ours only running at 50hz and not being as taxing on the hardware.
So now Europeans are seeing the US versions, they have slowdown they don't remember, but that has always been there in the US versions.