It is always written into a memory buffer first, which could be like 256 megabytes... it tooks time to fill it up, once it is filled, memory card speed becomes

It is always written into a memory buffer first, which could be like 256 megabytes... it tooks time to fill it up, once it is filled, memory card speed becomes a bottleneck. So, actually, writing only jpegs would trigger the slowdown later, so you could take more frames before the buffer fills up