I think it would depend on the capabilities of the USB reader. I have seen many USB 3 devices which are still using USB 2 internal components. Perhaps this is less so with USB-C devices but an inspection of the card reader’s specs would be recommended.
To do a quick check of the SD cards readers I have on my iMac Pro, I ran a speed test using Blackagic Disk Speed Test. The SD card tested was a SanDisk Extreme PLUS (32GB, SDHC, 80MB/s read, UHS class 3); not the fastest card out there but the fastest one I had that I could find quickly.
The internal SD slot tested at 52.1 MB/s write and 90.7 MB/s read.
The same SD card in a Satechi Aluminum Type-C Clamp Hub Pro tested at 50.7 MB/s write and 76.6 MB/s read.
This Apple KB article says that internal SD card readers on desktops communicate over the PCIe bus so tend to be faster than USB readers such as those in Mac laptops and external readers.
Of course, SD card type, class, etc. will also have a factor in how fast data can be read from / written to the card. Cheap, slow, older cards will be slow no matter how fast the reader can operate.