Deleting images on SD card is sometimes painfully slow, how can I speed it up?
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.