While recently shooting action and taking bursts of pictures I regularly filled the buffer, to me it seems as though it takes a very long time to write the photos to the card.
I have tested with all of my SD cards, they range from 30Mb/s to 90Mb/s, all are Sandisk. In each case I have shot a burst of RAW photos until the buffer is full and waited for the photos to be written to the card. On every card it has take 35-38 seconds to clear the buffer, is this normal or should I be concerned?
Check the front of the card for an icon that looks like a movie "clapper board" with a number on it. That number is the ACTUAL write-speed of the card (videographers need to know if the card can keep up with the video capture rate so they provide this number on many cards). The other numbers are merely the "transfer speed" (and generally you can READ a card much faster than you can WRITE data to a card.)
On higher end bodies you can use CF cards instead of SD cards. CF cards (especially if they are CFast cards) have a much faster write speed.
Here is a site that tested several cards in the 80D and there is quite a bit of variance in card performance: