I see than I am not the only one with simillar. I have GTX 1080 with latest nvidia driver. The problem must be somwhere in drivers <-> UE. I got a simple scene with some BP meshes on the scene. After a while I got some laggs every half a second or so. The strange thing is that when I press G to hide all the lines etc. everything is normal (120 FPS constant), but just when I press G to get out of Game View it starts to lagging.
The exactly same scene in 4.18 is working like a charm.
Our BP have some Billboards attached with simple material and texture. After I hide this is helps a lot but not 100%.