If you have any issues with FBX, have you submitted any reports? If not, please do this so we can see what you describe. Send the file in an email to firstname.lastname@example.org and include any relevant details (if beyond what you mentioned above).
I unfortunately can't send the FBX to you as it's a client project but I did source the problem. As part of the import test I converted 5 external .vrmesh objects (Trees) (Vray external reference files, much like Maxwell's MXS format). These were the root of problem with the FBX import in FormZ.
After they were removed the scene imports more rapidly but it's still far slower the the likes of Modo, Max, Maya & C4D (my import tests so far show FormZ to be 5-10 times slower).
Thea doesn't as yet have external file referencing enabled and FormZ doesn't provide for this beyond Components (which aren't fit for purpose for high poly vegetation, vehicles etc); so the only option the artist has for high poly vegetation, vehicles etc is to embed them within the FMZ scene file.
Back to the FBX import itself. It would be useful if at the very least you could provide a loading status progress bar (help manage user expectations with lengthy load times).
Also a more flexible set of import options would be useful. The animation and deformation stuff can be ignored from the options detailed below but the other options are very useful. In particular it's useful to be able to disable materials when importing FBX scenes that make use of 3rd party render engines. This way you have the option work with your imported geometry from 'ground zero' rather that attempting to import a bunch of material definitions that aren't appropriate for FormZ (or whatever render engine you happen to be using within FormZ).