Same here, I still had the issue here a couple of days back.Tried both 4.5.1 as well as 4.6 preview. Could be due to the conversion as you said. I have since moved on to reducing the number of grid meshes from around 1.5k to dynamically spawning 9 local grid meshes. That brought up the frame rates by around 60-70. v4.4 ran the level with 1.5k meshes around 35 fps. So it could be some memory/draw call issue with static meshes as well.