What SD card speed do I need?

For gaming, look at the A-class, not the big sequential number on the front. A reputable A1 card sets a random-I/O floor most retro and many PC-handheld libraries never exceed; A2 raises it but only shows real gains on hardware that supports it. U3 and V30 describe sustained sequential speed, which matters more for video capture than loading games. Buy a genuine A1 or A2 card from a trusted brand, sized to your library — and be wary of suspiciously cheap high-capacity cards, which are often counterfeit and fail when you fill them.