I own a 5D Mark III and I'm having some trouble getting it to shoot at maximum FPS (6 photos per second). Most of the time it seems more like it's shooting at 3 photos per second (just guessing). I just borrowed a 70D which offers 7 FPS and it certainly shoots faster than mine.
I have the 5D set to the following:
AF: AI servo
Drive: High speed continuous mode
I also use a UDMA 7 CF-card.
What settings may slow down the speed that I don't know about?
Then I will answer myself 😉
Battery charge makes a difference. With batteries fully charged it shoots faster. Also the AI Servo 1st and 2nd image priority settings makes a difference (focus vs release).