Why you have a FSUIPCConnection.Process() there?
What I should add is that, while that is working on the background, a timer, with interval 200, is also running at the same time. (processing data back and fort to/from fsx)
In the meantime I already got it fixed. by I re-creating the Quadrilateral class a bit and got away with it. I think, somehow, the FsLatLonPoint still wants to be processed or something, it is probably waiting in queue it looks like, I don't know,, after doing this I've got my results within a milliseconds.
Oh and btw, why do you do +90.0 and +180.0 on the lat/lon Degrees inside ContainsPoint() Method?
Thanks for your time though, well done on that DLL, very useful.