Huge perfomance hit, with validated (built) lights

We’re experiencing some really annoying perfomance troubles, in our project.

What we have:
-Medium size island, with few underground tombs.
-“Stationary” sunlight, “Static” skylight, and around ~12 stationary lights, inside tomb (all 12 lights have 3000 units fade distance).

After the full map Build - i’ve got a HUGE perfomance problems - around 12 fps, near the stationary lights (first Screenshot). If i’ll move away from underground - it’s still around 60 fps there.
BUT - if i select the Sunlight, and change “any” parameter, to “invalidate” the light-build (which is not changing anything visually) - i’ve got 110 fps, which is TEN TIMES faster.

How is that happening? The version is 4.23