Your not doing anything wrong, your modeling aircraft they way SHOULD look.
LOok at this.
This obviously looks ugly but with a little more detail
here and there, it can look better, this exports to FS
in just 1 second.
This however, has over 70,000 polygons like your models,
looks very very pretty and smooth, I don't know maybe
its somebody's intentestines or something, I just
drew up some splines and showed the mesh lol.
What most developers don't understand when they implement
this much detail is how MUCH data is involved in the texture
mapping, instead of a few vertices being defined for the texture
mapping data, you have thousands and thousands ... in DirectX
every time a part of your model is textured, this is a called a state
change, during that proccess, shaders may be applied to the surface and other effects that render a final image before the back buffer is flipped and you see the result 1 fully rendered frame ... this hurts the FPS real bad because your system spends more time doing rendering work but fear not...we live in an area of multi-core systems...I don't believe it will be an issue when you have 6 cpus working together to complete a task.
Hopefully Microsoft Flight takes advantage of this and we won't be limited.
By the way the model with 70,000 polygons took 18 minutes to export, I thought my gmax locked up but I just left it running, MakeMDL showed up after 3 minutes and after 18 minutes the export was complete.
In Flight Simulator I placed the object as scenery and made a flight, I did two exports, one for FS9 and another for FSX.
IN FS9 I got 13 frames per second flying about the scenery, NOT VERY GOOD considering I get around 25 on payware airports 13 fps from just a single entitiy is overkill, imagine If I had placed 4 of these things in the sky? FS would probably lock up.
In FSX the results where much much worse, I got 3 fps in FSX, I couldn't even take off because this was just not fun lol.
Anyway don't let that get your hopes down...people don't really care about FPS in flight sim, some folks are perfectly happy with even 8 fps, they love eye candy and taking pictures.
Really high frame rates are serious only when you want to have FUN, an incredibly smooth approach at 60 fps with all settings full blast will make you want to do it over and over again, the fluid momvement of the simulator is so realistic you get a head ache from it after a while...some people can't even tell the difference from 30 fps to 60 fps ... the human eye can't see this but you can feel the difference in response time when it comes to the performance of the simulation.
Try playing a first person shooter at 60 fps (the standard fps for games) you will surely admire the beautiful graphics and feel of the game, you wont want to stop playing!!
By the way I thought tSpline was only for RHINO? How did you get it to work with 3ds max?