It depends on the function. Many things will contribute. If your CPU can keep the cookie in cache then loading it repeatedly will relatively fast compared to hitting main memory. If your branch predictor can figure out the jmp over the int3 instructions quickly then that will also be fast. If the function is very short then the retguard stuff will add relatively more instructions to the function so will have a larger impact than if the function was long, etc.
I found that the runtime overhead was about 2% on average, but there are many factors that contribute.
I found that the runtime overhead was about 2% on average, but there are many factors that contribute.