Strange...   
For me the syria map is super smooth in VR.        
 Running on 1.5 PD on Rift S,  ASW enabled, MSAA 2x, and experimental "stereo Frustum Culling" enabled.     
 But I also play with only vis.range on medium and without ground shadows. (building and tree shadows)         
Edit:  Terrain Textures may have a big impact.  Big difference on fps between high or low on this map, as far as i noticed.