Canon stresses that you need a fancy, fast compact flash card to write data to the card as quickly as possible. The camera even supports a mode called UDMA Mode 6 which, according to Canon, is currently not even available yet on the market. The qualifier “UDMA” – regardless of the mode – is hardly used in the specifications of memory cards: manufacturers prefer to use terms like 20 MBytes/s and 200x (in units of 1.2 Mbit/s or 150 KByte/s after the 1x CD-ROM speed).
Need for Speed?
The point of fast cards is mainly that this allows a burst of images (4 frames/second) to be streamed to the compact flash card while the burst is taken. Thus the burst size would increase beyond the number of pictures that fit in the internal (SDRAM) buffer because that buffer is simultaneously emptied to Flash while it is being filled.
Leaky bucket model
If the memory card runs at p% of the speed of the data source, a simplified model predicts that the burst size increases by a factor of 1/(1-p/100) or 100/(100-p). This formula also apply to how much water you can pour into a “leaky bucket” until it is full: while you add 1 unit of water (data), p units have disappeared at the other end. For p=10%, the burst size becomes about 10% larger than for p=0 (an infinitely slow memory card, or non-leaking bucket). At p=100%, the internal buffer will never fill, meaning that the camera can write data to the memory card as fast as it is created. Your burst size is limited by the free space on the memory card.
Interestingly, Canon quotes the following disappointing improvements when you switch from a standard (8 MB/s) card to a very one: full RAW improvement 13→14 frames, sRAW1 15→15 frames, sRAW2: 20→20 frames. For JPEG the differences are quite significant: best quality 78→310. When the JPEG file size is reduced to 2 MB/image, faster memory cards give no improvement to burst length because a standard 8 MB/s memory card can keep up with the 8 MB/s (3.9 fps x 2 MB/frame) data rate.
The improvements you get with a fast memory card when taking still images appear to be minor (based on Canon’s data), and certainly less that would be the case if data could stream out of the SDRAM buffer at the bandwidth of the flash card. In general, it means that if you shoot RAW, you don’t need to buy a card that is much faster than 8 MB/s (53x) in order to increase burst size. And to support the 5 MByte/s video, you also don’t need more than an 8 MByte/s card.