Regular save edits, like changing learnable moves, changing possible abilities etc, shouldn't cause game lags. All that is likely the artifact of the mind, or the place you saved in (and then the game is trying to boot up and load that area).
For save editing to cause any potential game issue, I imagine you'll have to do some kind of crazy edit, and edited yourself into an area that the game can't load, or edited in stuff that the game struggles to load, while writing a bunch of garbage data into other sectors of the save.
Basically it all comes down to putting yourself in a situation where the game struggles to load all the corresponding data.