Creating a new level and performance is S-L-O-W

I’m trying to create a really simple level for a driving game. When I create a new level with just the basic settings (63x63 quads, Number of components 4x4, Overall resolution 253x253) and hit ‘play’, the preview screen is extremely slow (2 -3 FPS) and the landscape criss-crossed with vertical, horizontal, and diagonal black lines. Can anyone point me in the right direction?

UE4 version 4.26 with 32 gb RAM and RX3070.

Thanks in advance for any help!

-Dan