It takes some amount of time to save each image to the memory card. To allow for faster shooting, the camera has a built-in memory "buffer" which it can use instantly. This allows the camera to take rapid bursts of shots... but only as long as there is space in the internal memory buffer.
Once the buffer is full, the camera will say "busy" (and you'll notice the red light indicating that it is writing to the memory card).
As soon as it's clear enough space for just ONE more photo, it will let you shoot again... but of course there's only space for one shot. You have to wait for all the images to be saved if you want a completely free memory buffer.
You'll notice that intiially the camera is very fast in burst mode, shooting continuosly for several seconds and then suddenly slows down: click, click, click, click, click, click...........click...........click.
If this is what you're experiencing then that's the buffer going full.
The 70D can shoot up to 14 images in RAW format or up to about 65 images in JPEG format before the internal buffer is full and you have to wait on it.
This is normal. Every DSLR camera would have the same issue.
The problem is not related to the size of your memeory card... it's related to the rate at which data can be written to the memory card. Your 70D can use "UHS-I" class memory cards which tend to have faster write speeds than traditional SD cards. That'll help things write faster... but again, you STILL have a limited buffer size in-camera.
Every DSLR camera has a limited internal memory buffer -- although some cameras have larger buffers than others. The 7D Mark II, for example, has a large enough buffer to hold about 26 RAW images or up to about 103 JPEG images before the internal buffer is full.
Nearly every modern camera can take pictures faster than the fastest memory cards can save them. It's the memory card's maximum write speed which is the limiting factor... not the camera.
Tim Campbell
5D III, 5D IV, 60Da
