Test: Create a unit with very high health regeneration (say; 1,000,000,000 HP, 10,000,000 health regeneration).

While this unit is on screen, dota's performance will decrease significantly (framerate can go into seconds per frame instead of frames per second).
But, move your camera to a location where the unit is not visible and performance is fine again. So the likely culprit is a graphics effect related to hp regeneration.

I submitted a bug to the dota 2 linux bugs (as I tested this on linux only); but this appears to be a general issue. See https://github.com/ValveSoftware/Dota-2/issues/1341.
As it won't be likely to come up with the regular dota 2 game, the custom game section seems to be the best place to report instead.

Also; dota 2 does not check for limits to stats like HP or mana, resulting in overflow bugs if overflows happen (numbers >= 2^31).