editor lag on changing textures

I loaded level, no lag as I move around 4096 terrain, then I added ‘grass’ texture that comes std with engine, and lag right away, from 45 fps solid to 25fps as I move mouse, but very lag , way more than the 25 fps seems to indicate, and sometimes dips to 7fps.

Is just adding that 2nd texture enough to cause this ?

thx