6D focuses faster than 5D3 in low light ?

Will the 6D focuses faster than the 5D3 in low light with the 600EX-RT attached ?

It's allover the boards the problem I'm experiencing too. It's a shame that even the 5D2 in the same conditions, gets the focus faster than the 5D3.

Try for yourself 5D3 + 600EX-RT with AF assist beam, one shot, manual single AF point and you'll know what I talk about.