First, you sent the same request by email, and you have been replied to so, please, do not use both the forum and email to get support, because this would needlessly duplicate our support effort (we try not to have any question left unanswered), which will slow down support for everybody else.
So, make up your mind if you prefer to be supported by email or by forum, and stick to one method only.
As I already replied to your email (which assumed it was GSX that "slowed down" the sim, here you think it's the Addon Manager), nothing in the new update could have caused a slow down, and surely not a "dramatic" one. And, the Addon Manager surely CANNOT slow down the sim, also because it HASN'T been updated recently.
In the last update we remodeled some of the pushback tractors, to have better looking texture for the towbar, but it's a very small change that affects just one vehicle: I doubt it could have a visible effect on frame rate so, if you see any, it means your sim was ALREADY very close to breaking down because of your settings being too high and too many addons added so, it might be enough having a single larger texture to affect performances a lot, but that's because your memory was already almost full by *other* addons.
Since they only thing we changed with this update are those textures, you might try just lowering the texture resolution in the sim, try 2K instead of 4K or even less. If you see any difference, then that was the reason.
If, instead, changing the texture size doesn't seem to make any difference, than you can be sure your slowdown doesn't have anything to do with GSX, since those better textures for the pushback trucks are the ONLY things we changed in this update.
