Theoretically, an SSD has the potential to perform better. I got the 512gb steam deck+a 1tb SD card and for a while I was very careful about putting all the most demanding games on the SSD and everything else on the SD card. But once I started running out of space on the internal drive I just switched to putting everything on the SD card and I really haven’t noticed any performance difference. The only thing is maybe initial load times are slightly longer. But once everything is loaded and cached it runs basically the same from what I can tell.