https://wccftech.com/2021-macbook-pro-do-not-support-uhs-iii-or-sd-express/

Apple did not specify the SD card reader’s data transfer speeds on the 2021 MacBook Pro. While creative professionals and content creators will breathe a sigh of relief that this addition is present in the first place, it is also essential to find out how quickly you can transfer data between the SD card and the MacBook Pro. Sadly, none of the latest portable Macs support the UHS-III or SD Express standard, which may disappoint a ton of customers.

While it is strange that Apple did not list the SD card reader’s specifications for the 2021 MacBook Pro, all it takes is a little digging around, and we will have our answers. According to The Verge’s deputy editor Dan Seifert, both the 14.2-inch and 16.2-inch MacBook Pro models support the UHS-II standard, not the faster UHS-III or SD Express one. This means that data transfer speeds will top out at 312MB/s, which is still decent as you are getting near-SATA III SSD transfer speeds.