I use a Canon 600D with various lenses, and I currently use a memory card with 30mb/s. I always shoot raw and I've recently discovered that, if I shoot 4 - 5 RAW photos in a quick sequence, the camera refuses to keep taking pictures until I wait about ten seconds. Is this because the memory card is too slow, or does it have to do with the camera body itself?
- If the card is slower than the camera's maximum write speed then a faster card will improve the speed at which images are transferred to the memory card.
- If the camera's maximum write speed is slower than the card's maximum write speed then a faster card will show little to no improvement.
For more in general on how to determine how fast your camera can write data to a memory card and at what speed an even faster card will not matter, please see this answer to: How can I know what speed card to get for my camera? and Canon 5D Mk II shoots only at 2.1 FPS burst instead of 3.9 claimed, why?
In the case of your Canon 600D, Canon rates it at 6 raw frames at 3.7 fps before the buffer is full. Bryan Carnathan at The-Digital-Picture got 7 frames in his test with the camera set to the fastest possible settings (manual focus, manual exposure, widest aperture, shortest shutter time, lowest ISO, lens cap on).
The T3i/600D page at Rob Galbraith's CF/SD/XQD Performance Database no longer lists any data for SD cards tested in the camera. At the Camera Memory Speed page the closest camera that is rated is the Rebel T6s/760D. Canon rates the 760D for 7 raw frames at 5 fps. The 24MP of the 760D is 33% more resolution than the 18MP of the 600D, but the tested size of raw files at ISO 100 from the 760D are only about 15% larger than from the 600D.
This gives a data rate of about 54 MB/s for the 600D until the buffer is full but only about 15MB/s at one frame per 1.65 seconds (as tested by The-Digital-Picture) while waiting for the buffer to clear. In contrast the 760D gives a data rate of about 170 MB/s until the buffer is full when the rate drops to about 60 MB/s at 2 fps.
You have mentioned that you have a 30 MB/s SD card. But you have not indicated if that is the card's rated read speed, which is what card makers usually print on the front of the card and tout on the packaging and in marketing materials, or if that is the card's rated write speed, which is often much lower than a card's read speed. Also, please be aware that there are many counterfeit memory cards on the market with performance much slower than the cards they are sold as.
If your card has a 30MB/s read speed it is probably not writing quite as fast as your camera is capable of writing to it and a faster card may well improve your speed a little, but you're not going to get more than about 6-7 raw shots in a burst before the camera bogs down to about one frame every 1.65 seconds (about 3 frames every five seconds), so about 6 additional frames in the ten seconds you are now waiting for the buffer to clear.
Based on your report that you are only able to shoot 4-5 raw frames before the camera slows and no additional shots for ten seconds I would guess that your card is writing at a much slower rate than 30 MB/s and a card that can truly write at 30 MB/s would give you a little better performance.
This is a limitation of the camera, rather than anything else - when shooting RAW, the sensor output is faster than the camera itself can manage to process it into a RAW file, rather than being limited by the speed of writing that data to the SD card. Quoting from DPReview's review of the 600D:
RAW: 3.6 fps for 6 frames, then around 0.77 fps. 8 seconds to recover
which is pretty much the same as you're seeing. If you need a deeper buffer, you'll have to upgrade your body - or shoot JPEG, when you'll have a 40 shot buffer.