Iterative building lighting

Hello everybody!

I´m building the lighting of a scene, a small apartment, and every time I add something new I usually build the lighting - specially if I add a bunch of static meshes that I want to know how they´ll look like. Every time that I do so, I was wondering if the lighting calculated for the previous meshes that are unaffected gets recalculated as well, or if Unreal saves a little bit of time when doing so. I would say that the of course it gets recalculated, but I´ve been noticing that as of late the light builds don´t take so much time as they used to. And my computer is still the same, so I´m a little bit confused. Hopefully someone here has more insight than I do!