My gf is keto when she isn't stress eating rice. She tried to get me to go keto; like the grizzled sailors of yore I have shirked the siren call.

Basically, when compared to a keto diet it is less energy intense for your body to process carbs into muscle fuel. This fuel is what is needed to be able to go harder than say, z2. If you are taking in fuel that is less efficient to turn into the stuff your muscles use, you are giving yourself less to work with anaerobically.

My guess is that keto probably works OK for ultraendurance where you're holding a steady pace forever and ever, but is absolute garbage for anything resembling normal road racing or fast-paced group riding. While I am occasionally given back to back (to back!) bonk rides where I also avoid carbs, doing that on the regular is probably a recipe for training disaster.