Hey dude , sorry for answearing so late !!!
I’ve solutionned my problem with decreasing the LOD of 3D cesium map. And it worked 
Sure , you’ll have to adapt the LOD with the kind of project you do. Here , to obtain a skyline , my LOD is at 16 , but the first plan is , of course , at higher level . You don’t need to have a high level of detail for skylines. Only in close camera plans.
Hope it helps.