I would imagine that when the 550D was introduced card read/write speed wasn't too much of an issue as the high speed cards we have now weren't around (I got my 550D in late 2011 so it's pretty long in the tooth now). All I can find in the manual is a reference to the speed class for video, which is a minimum of class 6. My cards are class 10 and I would think it would be unusual now to buy a card less than class 10. I usually use a built in SD card reader to download my images and in my new pc it's very fast. Having said that, I use quite small cards so I'm not shooting 1000s of images on each card. Over 100 would be a very good trip. The raw files from the 550 are not exactly huge compared with the latest cameras. I haven't checked but the files from its latest incarnation, the 800D, will be significantly larger, and when (if) I convince myself to buy one for wildlife shooting I'll be buying it a new set of faster cards.