First of all you can switch iTR on and off as you want in the AF4 menu. So you decide if it is used or not. iTR does only work in zone, large zone and automatic selection. And yes, as stated in the manual, when iTR or iTR face recognition is switched on, it may take longer to focus then when iTR is switched off. However, no where is mentioned that the fps will drop by this. I never noticed that the fps drops in that case, but being honest, I never use automatic selection, maximal zone or large-zone.
Also on the 1Dx I never noticed that fps might drop because iTR was switched on.