Sorry for the pseudo-dramatic title.
I just wanted to share a small experience I had last night with P3Dv3 on my computer (i7 960 @3,2 GHz; 6 GB RAM, GTX970, Win 7 64). This is about the impact on performance that some graphic settings could have.
I have downloaded and installed the newest version of Orbx PNW which has been adapted to P3Dv3 finally. I made a first test flight with the default Bonanza and the performance was quite smooth. My graphic settings were quite high, and all of the little checkboxes for "send/receive" shadows were enabled. Autogen was on absolute maximum. Nevertheless, I was getting some solid 30 FPS average in flight, and up to 45 on the ground, which is quite good (FPS is in unlimited mode).
Then, I proceeded to install the two OrbX airports I had for that region: Concrete and Darrington. They are not adapted for v3 yet, but it's easy to install them into another folder then copy-paste everything in the correct place again. It worked perfectly and of course I didn't forget to reinstall the latest OrbX libraries (adapted to v3 already). Then I restarted the same flight I did in my first test: Concrete to Darrington, following the valley East -> South-East -> South, quite easy with nice weather (building storms theme). In the starting airport, Concrete Mun, the FPS were very good, around 40 to 45 FPS on the ground. Then, from takeoff the FPS went down to a stable 30 FPS, which was smooth enough to really enjoy the low altitude flight.
However, things got bad when I got close to Darrington. On approach, the FPS slowly crawled down to 11 FPS only, when I was on approach to runway 28, facing that mountain behing the city. At that moment, the city and the airport were NOT visible on the screen. I could see only the montain wall covered with trees. I hit pause, and the FPS stayed at 11. This means it was a graphic problem, some setting was killing my video card. I tried the most obvious choice first: I deactivated ALL of the shadow checkboxes (send/receive)
The FPS went up to 75 !
Then, I reactivated all of the shadow checkboxes, excepted the ones for the trees and the very last one (can't remember the name): the FPS went down... to 45 FPS
Bottom line is: the shadows for the trees can kill the performance, even with a GTX970, in extremely dense autogen areas. The shadows for the buildings might have an impact as well, I didn't check that, but this will be my next test when I will fly over a big city. Seattle might be a good destination for that.