If you mean slide to power off as shutting down, I never do this. I just use the on off button on the side to turn it off and then back on again.
Why would anybody use the slide to power off all the time? I only use that perhaps once a month, unless you are just asking for the Slide to power off and then on again time it takes.
If I use the slide to power off and then back on again, it would take me 3 1/2 to 4 seconds to get to the Apple logo than a few more seconds longer to get to the Face ID or the touch pad to enter my passcode.