Contrary to what you may have been reading, you don’t need a fast memory card to record video. Actually, most cards manufactured in the last few years should be fast enough since the data rate required by H.264 video is much smaller than what is actually used when taking pictures – as long as it is greater than 30MB/s (see below) – note that is usually also seen as 133x – so anything greater than 133x speed will be fine.

There have been some comments that you can get some artifacting in your video if you use slower cards, and I asked Canon about this when the Canon EOS 5D Mark III was announced:

is there a ‘minimum’ card speed recommended for the Canon EOS 5D Mark III to handle the All-I compression?
CW: We recommend using memory cards with write speeds faster than 30 MB (not Mb) per second for the ALL-I setting. The faster, the better.

What about the buffer warning icon then?

This icon appears when the camera buffer is filling up too fast, meaning it can’t keep up processing the footage (ex: too many details to encode), not that it can’t copy it to the card fast enough.

Still, fast card are good because…

The benefit of faster cards appears when it is time to transfer the footage to the computer. A fast card (plugged in a fast card reader) can empty the card up to 5-6 times faster which can be a real time saver when you have to empty 4 of these at the end of the day.

Additional info

B&H Photo/Video has an in depth article on memory cards

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