IRS no longer processes returns in first in first out batches. They are processed individually so returns sent in at the same time will not necessarily process together.
The IRS states that 9 of 10 returns will process in 21 days or less from being accepted not filed. However considering that the IRS processes more than 150 million returns that means 15 million will be delayed for more than 21 days.
Think of the IRS process as a long pipe line. Some returns will be rejected before being allowed into the pipe and the rest will be accepted. Then once accepted into the pipeline many will process directly through without any delays and some will not. Some returns will be siphoned off for further review of some kind. The basis of what triggers these reviews is a well guarded secret but some are just picked randomly every year.
Some reviews are automated and some will require human attention. The processing times will vary due to the continued understaffing situation at the IRS.
