Does it take long to grade a scantron? No. But there are a lot of other things that go into grading the test that need to be considered. So let's take a quick look at those things, using round numbers.
Day 0 - Test
Shipping the test sheets from the 100+ test sites to NCEES in South Carolina. - 5 days, which includes weekends and "ground" shipping from remote locations like Guam, CNMI, Hawai'i and international locations.
Actually running the 20,000-30,000 scantron pages that arrive - 3 days
Manual double check of the scantron grading of the tests. Lets say that they manually check 5% of the scantrons vs the total. This is still a 1000 pages - 5 days
Statistical analysis of the results to identify anomalies; collecting reports of weird questions from around the country, investigating incidents of potential cheating, etc. - 14 days, including weekends.
Assembling test exam committees to review anomalous questions and other statistical quirks, determination of cut score for newer tests based on analysis, and disposition of various items listed in the previous entry. - Probably overlaps with the above to a certain extent, but the smaller exams always schedule their exam meetings late in the grading cycle, so excluding overlap, lets give it 5 days, but more is possible with a new and smaller exam
Miscellaneous administrative work, including uploading results, QA, management approvals, and communicating results to the Boards - 3 days
Add it up and you get about 35 days, which is consistent with the April release times. Add a few more days for the November holidays and you get something closer to the 40ish days we tend to see for the October results.
You can argue with my assumptions for durations, but the turnaround time of slightly over a month passes the sniff test.
*Someone else posted a similar analysis a few years ago but I can't find it. Hence I'm recreating it here. If someone has the link, please post it.