I made a little image to better picture what's going on with optimised PAL games, both then and now. It's slightly more technical than this, but this is the basic gist.
In the top row we see the original American release, displaying frames 1 to 20 perfectly.
The second row shows the original PAL SNES release. To get it matching the same speed, they just drop every 5th frame. It's not really all that noticable because the raw output of the SNES is 50hz. There's nothing to account for. It looks okay and plays okay.
The third row is what happens when we take an optimised 50hz game and play it on the Wii U. Because the Wii U can only
output at 60hz, this means a 50hz source has to repeat a frame every so often. But because the optimisation already dropped a frame, and now we're repeating one to make up for the hz difference between source and output, we get the effect people are calling "janky". I love the term to describe it.
You can see, the 4th frame is now displayed twice, then the next frame is America's frame 6. If we had the US ROM, we'd just go 4 -> 5 -> 6, but the U "breaks" these optimised games and we're forced to live with 4 -> 4 -> 6. Even without playing, you can imagine this is not ideal..