You stand still for up to five seconds, even though there are no vehicles nearby, until clearance is given. Gaps between vehicles are ignored, even though 10 vehicles could fit between them.
Ideas
This has happened to me in back to back races in 2 player career. First in Bahrain then in Jeddah. I go into the pits ahead of the next car by 4-5 seconds, the pit stop doesn't have any issue (optimal turn in, 2.3-2.4s stop) but then I'm held for no reason and after 6-7 seconds all the cars that pitted behind pass before I'm released. I am playing as Kick Sauber if that helps. This has cost me 2 races in a row and it's making the game unplayable now.