One of the best times I ever got in a TT was when I turned up late, AND took a wrong turning on the way to the start, so my warm up was a 15 minute ride over partly gravel roads at a NP of 92% FTP, getting two top10 Strava segments on the way, and arriving 45 seconds before my allotted start time.
Not sure that’s necessarily a protocol to follow in every scenario, though.