Maybe it’s just the game but after you get all the upgrades to fit door panels no matter what you do you can’t supply enough resources. Maybe it’s a bug, maybe it’s the game. I don’t know. I have a suggestion to have two slots for fit door panels. Like interior fit door and exterior fit door.
Just a suggestion.
It can be a bottleneck, but then nobody ever said you cannot branch and then re-combine the production line to get around this 
Also…not all cars have to have all upgrades. if you interleave cheap cars with simpl;e doors with more pricey cars with all the options, you will find the bottleneck less of a problem.
There is a bug in here somewhere. I have two Fit Door Panels slots, right next to their own dedicated Resource Importer, with a few MAKE DOOR PANEL slots in the same chain. On Game speed 1, no problem at all. They run beautifully On Game speed 3, they spend a lot of time in Waiting for resources mode and end up blocking my whole line. I’m running at nearly 60% but when I go to speed 3, the efficiency drops to below 50% with all other things being equal. It is very obvious on my efficiency graph when I run in speed 3 vs speed 1.
We’ve been discussing that over here,
But I noticed it as well, when you switch to hight speed, the panels jam up, and even back up the line, then switching back to speed 1, the line will balance itself out over time.
I have to do more testing in 1.28 / 1.29 as there have been optimizations done on the resource pathing.