To an extent permitted by law, which is why they have experts who adjudicate these matters through the MEB process and not the promotion board process.
The reason it is taking so long is upon review at HAF, it was noticed that someone people who received a P from their commanders probably should not have when their record contained several QFIs that would otherwise make them not qualified to serve in the next higher grade. Instead of attaching a PRF and/or sending a PRF, commanders just used this "new" process to slip them in. As such, HAF went through every record to make sure those who were promoted actually "deserved it" based on members' OPRs, medals, TRs, and the OSB.