Aren't modern type 10 MicroSD cards plenty fast enough to serve up video? How much faster do they need to be, and what applications would require it? I though

Aren't modern type 10 MicroSD cards plenty fast enough to serve up video? How much faster do they need to be, and what applications would require it? I thought RAM (typically 2 or 4 gigs these days on most phones) is where the fast computation takes place, whereas the non volatile memory merely emulates disk storage.