One side note - if you look at the Objective Raids and similar sources you see that on nearly no assembly line all parts are produced onworld. That means for each mech you have to import critical parts from several other worlds sometime many jumps away.
From my understanding the crush on the interstellar trade with the Amaris Civil war and later the SWs is one major reason why production numbers of existing lines decrease.
You should still be able to get similar speeds as the original, as long as you have a steady supply of components to be fed to the proper locations. I.e. imagine a Marauder plant that used to have a linked PPC manufacturing center where the PPCs were built in a separate building and sent via railcar to the main mech assembly area. However, the PPC manufacturing center got destroyed. So now the owner of the factory has PPCs shipped in, a basic crane system used to load PPCs onto railcars, and the railcars come in are at the correct rate. If the supply of PPCs gets disrupted, the factory will slow down/stop, but as far as the main assembly line is concerned, it is still receiving PPCs from the subfactory.
Of course, you might have a small file receiving errors from the PPC factory monitoring software (main factory can't make contact with PPC factory), so it adds a single line to an error file. Each time it the factory builds a Mech it has another line in that file, and this causes a gradual slowdown in production (all of the error files are reviewed before commencing assembly, and the factory engineer know to press the 'acknowledge' prompt to get the factory going for each Mech). So over time there is a gradual increase in time for each Mech to be made, too slow to notice, and nobody knows why (secondary diagnostic building was cannibalized to keep Mech factory going).
The other fun is a factory where a printer buffer problem is just waiting to show up. (Short version. Automated factory in US suddenly halted all production in middle of the day. C-level types, Main Engineers, and similar high-ranking people are immediately called with orders to FIX IT! On way by one of the interns spots a printer that is out of paper, and fills it up. Printer begins printing again, and within a couple minutes the factory just starts back up again. Turns out the factory computer wrote reports to that printer, and the printer ran out of paper. Reports filled up printer buffer, until buffer was full. Factory stopped until room was available in buffer. Once the printer started printing, the buffer soon had room, and the factory started back up again. New standing instructions were to make sure that printer always had paper.)