HPL Level Editor start "lagging"

[The chaser]

That depends on the level. If you aren't looking at all the stuff, it goes fine, if you are, it will lag a bit.
Not a problem if you toggle culling fog while in the development, or using clip planes.

Culling is not rendering the objects that the player doesn't see, to increase performance. You can activate the level fog, and toggle "Culling". That will hide the zone covered by the fog, making you unable to see it and therefore increasing performance. If your level doesn't have fog, just make it with culling fog and then, when the level is done, disactivate it.

If it's a really big map, you can use clip planes (At the bottom, at the center).