As bad as this sounds, I think certain companies are delaying the process in order to move back payments. If you know the schedule of the MSC's payments and you are doing a shop that is close to the deadline for a payment, chances are you will see it take longer to be approved, just long enough for the deadline to pass.
To be fair, I am not saying this about any specific MSC, just in general as I have seen this trend more and more lately.
To be fair, I have also see the reverse, something what usually takes a few days to process gets pushed through faster so they COULD include it in the deadline. It all depends on the MSC. Heck, last week I did a shop I had done the previous month, everything was the same, and last month it took 8 days to approve, this month it took 15 minutes (YES minutes). I could not believe it, I submitted my report on a Saturday evening and 15 minutes later I received an email from the editor thanking me for a perfect shop and that he approved my shop for payment. I logged in as I could not believe that and sure enough it was marked to be paid.
So it works both ways.