Wiki says that SD 133X is capable of 19.5megaBytes Read, and 19.5megaBytes of writing speed are theoretically possible. So it appears that 20 mega Bytes per second would be the maximum the buffer could be cleared in theory.
This probably isn't apples to apples, but my 5dmk2 has a 13 shot RAW buffer, and clears out in about 3.5 seconds using a 600X CF card. Each image is around 20MB, so that's 260MB/3.5(sec) = 74.X MegaBytes per second of write time. So in relation it doesn't appear that 20MB per second is very stellar. On top of that, the 5D3 has a much larger buffer, so one could potentially be waiting much longer for the buffer to clear.
It puzzles me to no end why Canon would offer dual SD, and yet manufacture a standard (not high speed) SD slot .... Head scratcher***