The last 2 times I've flown out of DCA (Wednesday before Thanksgiving and this past Friday), the C gates security has been a zoo in the morning. Pre-check line was closed and the main line stretched back to where you enter from the parking garage/metro. Both times, I've gone straight to the B/C security and used the PreCheck line (no wait) then take the bus to C terminal. So, if you are flying out during the holidays and have PreCheck, I would suggest doing the same thing I did if C security lines are crazy (probably saved 30 minutes or so)