I'm sorry, but I can only repeat and confirm nothing in the Addon Manager can cause such fps loss, as confirmed by other users in this same thread, which fixed it by properly upgrading P3D 4.2, which wasn't fully updated, possibly with some mixup of files between the old and new version.
As I've said, fact you see this only with the Addon Manager enabled, doesn't mean the Addon Manager is a problem: it only means the Addon Manager is SHOWING you a problem you already have with your P3D installation, that no other addons can show, because they don't use some advanced features in the new SDK, which we use to IMPROVE fps, instead. But we are using this method since several months so, it cannot possibly caused by a recent update.
So, be absolutely sure you updated P3D correctly. If you updated just the Client, try updating the Content too, because this what fixed the issue for some users.
Do you have a monitor with G-sync perhaps ? I've noticed that, if you enable G-sync for Windowed applications, the frame rate collapses, but this happens with every program, even FSX (when run in a Window), but it always affects P3D, since it doesn't have a true Full screen mode. And, it happens even with no addons enabled, not even the Addon Manager so, it might not be the cause of your fps loss, but it's worth checking it.
And, of course, always use the very latest version of the video drivers. It's possible a BAD tweak can cause this so, try to reinstall the video drivers making a custom "Clean" installation, which will remove all non default tweaks.