I'm using SanDisk Extreme in all of my hacked Switches and, honestly, I'm yet to run into a major issue. Slowdowns happen from time to time but are usually limited to loading screens, when game's switching between in-game footage and cutscenes and if games are poorly optimized for the Switch (I tend to avoid these and, if available, I play them elsewhere). Thankfully, first-party Nintendo games usually run well enough for my taste, which is what I mostly play nowadays anyway.
Still, you're probably installing more titles than I am so I could be that I'm yet to run into an issue. ^^