Umberto, I ran your test in the same position with no 2D panel and a default plane. I was getting 61FPS.
That's very good, and at least shows that what pertains to the scenery, performs well. Note that, it's possible that, if you had your vertical sync locked, since that number is very similar to the monitor refresh rate, it's possible you might even go faster than that, by disabling vertical sync. If was already disabled, than it's fine too, 61 fps is surely good.
Now, starting from there, you might have some ideas how the fps goes down. For example, using a 3rd party average complex airplane, then the PMDG NGX, both in 2D and VC, and you might seeing some pattern.
For example, I see a big difference with the PMDG between 2D and VC mode. It's much faster in 2D, which is a shame, considering the VC is that airplane strong point (but it's really a monster, with more than 1000 drawcalls).
The image posted by Silverbird, shows the PMDG on approach, from the VC, at 42 fps, which I find to be very good, what fps you get in a similar situation/position ?