In reviewing the code for post-processing, the only thing I could figure is that the original devs were using it as an antialiasing substitute (probably for early video cards that didn't support it back when it was originally developed). It appears to simply take two back-to-back frames and blend them together. It reduces edge aliasing, but at the cost of making the entire scene slightly blurry. When I added the feature to enable AA, I disabled post-processing if you select it because their post-processing doesn't appear to do anything else than what I described. So it's not really post-processing like newer games use it.
As for the slow ship, I can't really answer since it might just be your character stats and the specific ship (and its load) that you are using. You can send me a save and I can check things to possibly determine why. But I have no recollection of unduly slow ships when I start new games in ERAS myself. I also don't set the ship stats for ERAS, as that is handled by MK and I simply post up what he deems they should be.