Unless you're a working video editor or only play 4K videos, the reality is that even older I/O speeds will probably be adequate 98% of the time.
In my case the most intensive use is to run six media player windows at once and then task-bar selecting "Show Windows Side by Side" so I have six beautiful women dancing before me.

Remember: your OS, apps, etc... are probably going to boot and run from your internal drive
Yep. OS speed doesn't seem to vary at all because of this. But I *do* need my external drive to load those six videos of Abbi Secraa and play them without stuttering.

this feels worse w/USB (where I'll start a backup and it writes at USB2 speeds but degrades to half that within a couple hours).
Oh yes I think everyone can relate. I still use a standalone MP3 player, and connecting it via USB and adding songs to it is so s-l-o-w sometimes, I am reminded of the days when I was using a 1541 drive on my C-64 and had time to paint the room while waiting for things to load: