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.
In my experience, it has been surprisingly difficult to line up:
• (part 1) a fast card with regard to a particular camera (or slot on a camera)
• (part 1) a “reliable” or “robust” card … e.g., at least three SD cards I’ve had have simply broken; one in camera (essentially ruining the camera, as the cost to fix it/extract it exceeded the camera’s value)
• (part 2) a fast USB interface between the card and the computer
• (part 2) an interface that includes both CF and SD slots
• (part 2) a short cable / compact enclosure
Hi… I would go with the Sandisk from Apple, since they have a 20% price reduction atm, it’s a reliable manufacturer and as Apple are selling it you can be pretty sure it will work well with their machines.
Since you also want to attach external disks (which I presume you have and are not USB-C) you might want to consider whether a multi-function adapter could better suit your needs, for example this. More expensive, but maybe less fuss. Note it says ‘coming soon’; there will be similar from other manufacturers.
You can usually find external readers specs on the manifacturer website (in it’s not a generic chinese brand, I guess): I use this one from Lexar connected to a thinderbolt 2 hub, and it is well suited for my needs.