Did you delete the exact same things off the sd card on each of the computers?
If you were deleting all the stuff from your sd card. Did you select all then click delete, or did you just format it?
Also what sized sd card was it? something like a 32GB?
Anyway, I'd format your sd card, then get a few files like a couple of films, and a few mp3s. Stick them on the sd card and then select them and delete them, and see how long that takes. Then format the sd card again, and stick the exact same files on it and try deleting them on another computer.
If they delete around the same time, then I'd guess maybe you had some corrupt data or something on your card before. If it's still slower when deleting on your mac, then I guess it's something wrong with the mac. Maybe a dodgy usb port, or the drivers (assuming you are using a usb sd card reader). Maybe try sticking it in a few different usb ports to see if that makes a difference.
If you aren't using a usb sd card reader, maybe try 1 and see if that makes a difference.