How do you tell if the aircraft has collided with said cylinder?

Well, how do you tell the aircraft to collide with a solid house? Come on, if all the houses would not have a texture wrapped around and LODs and smoke rising from their rooftops and more complex structure than a simple block, the impact on the framerate would be minimal. Same should be true for trees... At least my old E8400 can easily handle building amount on "high" when the building detail is on "very low". So I guess it could still handle a solid tree with actually no detail in the structure, just an invisible cylinder... Well, oviously it is not that simple, I am sure they tried something... I was just wondering, that this is maybe a russian issue, as DCS seems to be not capable of having solid trees in their engine neither, lool (just kidding...)

As someone already said: Use a graphics primitive or two, as the "collision zone" (modelling every twig per tree would indeed be taxing to your system's resources and it wouldn't matter anyway). Since WWII planes don't move at warp speed, process only the trees/primitives in the immediate surrounding of the plane*. If (plane inside collision zone) then {activate Damage model};

* maybe a bit more to get a safety buffer for system- and/or online lag. Be aware, however that A = r² * Pi. You could further optimize the thing if you only consider trees in a "wedge" in front of the plane, with the wedge's dimensions chosen that it safely covers the plane's flight envelope, at all times (even when shot down and tumbling and such things).

As Luthier already said, even with a single primitive it currently takes too much CPU time.

I agree if the whole of the Channel map is covered by 'active trees'

when COD is ready everybody will say
PLEASE MAKE RISE OF FLIGHT LIKE COD:!:

+1
I think its just Laziness, on there part

Fixes have already been suggested. Would not be a problem if they had not misused/used the completely wrong system for tree generation (speed tree).

which is a prove the sim is not optimised well...

All you post mate is diatribe, utter nonsense and whines. If you have nothing to contribute shut the hell up!! Sometimes it's better to say nothing than to say something stupid and make yourself look like a complete tit!