trash empties very slowly from SD card

First off, if you reformat the cards to Mac OS Extended then you won't be able to use them in your camera. That format has to be done in the camera - which is also the much faster way to erase a card: reformat it with the camera. The slownesss is pretty much inherent in the fact that the camera cards are FAT32.

Make sure NOT to use any 3rd party anti-virus apps that might cause problems like this (macOS built-in protection is all you need anyway). Also 3rd party "cleaners" should be avoided because they often corrupt the system.

Lee Miller2 wrote:

This is a university machine, for which they load on some additional security and administrative software. It strikes me that interfering with trash emptying, but not read/write/delete operations would be unlikely, but I suppose not out of the question.

Such organizations might be forced to use such unnecessary and harmful 3rd party anti-virus software on macOS.

One faster workaround might be to format the SD card if all its contents are deleted anyways. Or, better yet, just format the SD card in the camera which best knows what formatting options it supports.

If you are using a built-in SD Card Reader on the Mac, then try using an external USB SD Card Reader instead to see if that makes any difference. I have seen multiple posts on this forum where some SD Cards have problems with the M-series Macs' built-in SD Card Reader.

You may also want to check the speed of that SD Card using Black Magic Disk Speed Test available for Free in the Apple App Store. Flash media tends to slow down their write speeds when performing sustained writes. This may or may not show up in the Black Magic speed test depending how long the write portion of the test runs. You will need to make sure to select the volume on the SD Card as the destination for the benchmark test since it defaults to the boot volume.

Also, disconnect all other external devices just in case one of them is causing a problem or interference.

What’s the speed rating on that “High Speed” SD card?

https://www.kingston.com/en/blog/personal-storage/memory-card-speed-classes

Irrespective of the “high speed” name, “high speed” is the second slowest SD available, and is the slowest meant for use in digital cameras:

https://en.wikipedia.org/wiki/SD_card#Speed

Emptying trash likely involves sending an erase (TRIM) command to the storage, and flash erasure is slow.

Erasing the whole thing will be faster.

Try a faster card?

200 Mbps is roughly UHS-2 and 20 MBps, well faster than the older speeds, so yeah, probably not the card.