Why unit upon height/cliff reduce FPS a lot? (esp. w/hp bar)

This maybe sound weird, but I experienced that once lot of unit is upon height or cliff it will reduce FPS a lot!

Let's see...
First I try to use initially cliff (at 2) I almost same as no unit, FPS always 60-65
But once I try to lift up a cliff for example, well, now FPS is reduce to 39 when showing health bar. (about 55 FPS without health bar)

See my image for little example.





This maybe a little problem but with map like Darkest Hours of Guardians, its reduce a FPS to 25 or less...

Any solution?

Don't use cliffs, but use the terrain tool to make it higher?
Don't know if it helps tho

Nope, as I said, it's both Cliff and Height (title said).
In image which use cliff is just quick referring.