I'm not sure what do you mean with "previous version", this is the only version that would ever work in P3D V5 and, of course, nothing in GSX could possibly cause a loss of fps, nobody has reported this and, in fact, most users see a SIGNIFICANT increase on fps, when using P3D V5, especially considering in V5 we removed Render To Texture feature, used by logos and jetways number, since we would have to rewrite in Direct X 12, so GSX in P3D V5 takes less VRAM and renders less stuff.
The fps drop when undocking displays is a known problem, which happens also without GSX.
In P3D V4, IF you were using TWO different graphic cards ( not just two monitors ), you could fix this by disabling Render To Texture in the Addon Manager, as explained here:
http://www.fsdreamteam.com/forum/index.php/topic,23780.msg158457.html#msg158457But, as I've said, that option in V5 is completely disabled ( not just disabled, there's no DX11 code in the V5 version of the Addon Manager ) so, it's not the issue here.