Processing load is too high.
what hardware you rocking? and what are you trying to run?
both of the high end demos (the valley and the city) have their requirements listed in the editor startup map or the documentation. you have loads of geometry or A.I. so a beefy cpu and loads of ram is necessary to shovel the data around or compute the A.I.
for humble and moderate game development a 6 core cpu can be enough. you require a receent gpu (preferably rtx 30x0+) with atleast 8 gb of vram (more is always better) or you have to find ways to run it with less. i can confirm with the right setup it runs stable on a 6gb card.