uite a few that sounded like a similar problem to mine and the guys in this post, but the first thing you seem to start by saying is that its not an FSDT or GSX problem. Had you then gone on to suggest disabling the RTT then that would have saved my a lot of messing about, including uninstalling GSX and then reinstalling.
You don't say anything about your configuration.
Assuming you use more than one video card, as I've said in my previous message, this is a known problem which cannot possibly caused by the latest GSX update, it has always been there since P3D4 came out, that's why the RTT feature can be disabled, the option to disabled it it's not new, because we know there are issues with more than one video card, but nothing in the GSX update changed anything related to it.
The only possible explanation I have is that, while the usual flickeriing problems that were associated with SLI cards, in a recent P3D update ALSO started to cause low fps, when multiple cards are used. You must have noticed it only after a GSX update, but it was only a coincidence.