Very slow transfer speed on SDXC card, iMac 2017

Using SDXC cards for additional storage is not wise. It would be much smarter to buy either an external HD or SSD. For an external SSD I would recommend the https://eshop.macsales.com/shop/owc-envoy-pro-sx. Using SDXC cards is dangerous as they can fail easily and yes, they are slower than SSD storage. This would be a huge upgrade in your workflow. You may want to consider buying 2 of these and use one for your startup drive. Doing so would be a HUGE upgrade in performance for a 21.5" 2017 iMac. You can learn how to do this in How to setup an external SSD as your startup disk

I second the idea that it is not good to rely on memory cards for long-term storage.

As for why card read speeds are low, it's not just the card that matters, but the reader. If the card reader is a USB 2 model, or is internally connected to a USB 2 bus, that's going to limit things to USB 2 speed. If the card reader does not support UHS-I, then when you use a UHS-I card, the bus interface will operate in one of the older, slower modes that both the card and the reader support.

https://www.sdcard.org/developers/sd-standard-overview/bus-speed-default-speed-high-speed-uhs-sd-express/

Your iMac has USB 3.0 ports, so if you got an external USB 3.0 card reader that features UHS-I (or better) support, you might find that you could read your cards a whole lot faster using it.

----------

Other things that can slow down transfers are the speed of the drive to which you are copying the files, and Finder copying overhead (if there are a lot of small files). The overall speed of a transfer will reflect the weakest link.

Is your machine a 100% internal SSD or does it have a Fusion Drive. If it has a 100% SSD then yup it's fine, if it is a Fusion Drive nope no such much.