For a crit, you need to do a couple of maximal efforts in warm up. My crit warm up is 15 minutes easy, 5 minutes tempo, 5 minutes easy, 2 minutes steady state (LT), 2 minutes easy, 2 minutes steadystate into a power interval, 2 minutes rest, 2 minutes steadystate into a power interval, go race.
If you' haven't gone 10/10ths in warmups, the first time you do it out of the first corner really sucks.
For most short crits, the risk of exhausting yourself is very low, and the risk of being underpreapred for early hard efforts is high.