To check for holes, go to the track in CM (not available in the free version) and click the arrow next to "update map" at the bottom > "update map from surfaces". You can check/uncheck various surfaces, screenshot it and for clarity I isolated the road and painted it red in Photoshop; just so I knew where what part is on track.
While ingame, open the "Render stats" app (this might be one of the developer apps so it needs activation in CM Settings > Assetto Corsa > Apps > check "Developer Apps"). Overall it's not very well documented but it has been covered in one of Stefano's live streams once, explaining some of the numbers.
For performance, the red graph is quite important. If that spikes you have some issues. Also the amount of draw calls (DIP) and batch size (B.SIZE) are important. There is a bit of explanation
here.
DIP is heavily influenced by the amount of materials in this case. With CSP's "Object Inspector" app, you can find out what material is used, which textures are used, etc.
There are atleast 4 different materials for grass that use the same settings; that should really be one material.
Also, everything you see on Kunos tracks (and most optimized mod tracks, like LAC) is not used for physics calculations. That's all done on hidden mesh. In the case of this Magny-Cours the entire visual mesh is split into sections to be used physically (the objects begin with a number in front); adding to the DIP count and reducing overall performance.
In the end it can run fine on some systems. But it can "kill" some other systems.
And regarding that: Sol advises to use some settings. But CM has so many more settings that affect performance. Think about GrassFX, realistic mirrors, tyre deformation, heat haze, ambient occlusion, etc. Add them all up and it can drastically reduce performance.
