Valve’s pre-release testing of handheld devices shows that in some games, micro SD card load times are nearly as good as SSD load times. If users use a large-capacity SD card, they will hardly notice the difference.
Canadian slut king Linus and tech video channel The Phawx both tested it on Steam Deck and found that in games like Control, Dead Cells, and The King of Fighters 5, micro SD and SSD load times were almost as fast as the same. In some cases, the micro SD’s loading time was only 2 seconds slower, while in other cases the test speed of both was exactly the same.
Portal 2 and Ghost Walker are about 10 to 17 seconds slower on micro SD though, so actual results may vary widely depending on which games Proton software is and Steam Deck hardware compatibility. Currently, Valve only allows testing of a few games, and the Steam Deck software is not yet complete.
In the dismantling video of Valve, the official said that even if it is completely feasible, it is not recommended that users replace the internal SSD by themselves. The official said in the video: “Our SSD is very close to the wireless module and has been specially selected and tested to not interfere with Wi-Fi and Bluetooth.”