Deleting images on SD card is sometimes painfully slow, how can I speed it up? I have a Samsung Galaxy S3 with about 3G available storage on the device, and 40G available on a SD card. The Camera settings are to store photos on the SD card. When using the stock Gallery app to browse photos, deleting them is often painfully slow. When it's working properly it takes less than a second, but when it's not could take maybe 20 seconds or more.

What could this be caused by? What could I do to diagnose and fix the problem?
I remember this issue.
It's not a problem with your SD card or your phone in general, but with Samsung's TouchWiz.

The system is completely overloaded (800+ mb RAM out of ~900mb used right after boot), so many apps are slowed down when requesting RAM.
Asking the system to delete a file (because of taking care of permissions, handles etc.) requires the system to wake up a process (or launch it if necessary) which then asks for it's own RAM and thus needs to wait until the system cleaned enough memory for it.
Because of the tight RAM condition, it takes time to close processes that are not needed anymore.