The VA does the ratings, and generally they receive your packet before the PEB makes a finding. The timeline the Military and congress and the rest of us want for a rating decision and the time it takes are just not gonna meet.
So a explanation on why it takes patience: The VA has to review your entire medical file before making a decision. The meat of it is fairly simple, just go off the DBQ and input it into a computer and out comes a rating. That leads to problems sometimes, so they have to go over every piece of paper and make sure everything makes sense, and where it doesn't, make a decision on which information works better.
So yeah, maybe that'd take a couple weeks, but there are claims coming in from all over. So they have to do the ones who come in first, first. Its not a problem they can just throw people at either. VA says it takes on average 2 years to train a rating officer to be effective. You can't just hire any old guy off the street either, there is a pretty high level of trust they need to place in these people. They're deciding on where monies go, but also they have to read all the intimate details of your medical file.
If you get too frustrated, just remember, the average time for a claim to get a decision outside the IDES process is over a year. Every once in awhile the media latches onto a story of a guy waiting over 10 years for a decision. It wasn't that long ago that the average time to be in the MEB process was over a year, and I certainly knew a guy who was in the MEB process for 3 years.
I would say the average time now adays from NARSUM signature to rating returned to you is 4 months, +/- 2 months. There are still some that take over a year, but they're becoming more rare. Its a far from perfect system.