With Eureka 1.1.0 beta.8, Kotlin for Forge 3.4.0, Oculus 1.2.5a, Rubidium 0.5.3a (why is Rubidium in an unfinished version but Oculus, its addon, isn't? wack), and Valkyrien Skies 2.1.0 beta.10, the game runs about as expected.
With the addition of EntityCulling 1.5.1, FerriteCore 4.2.1, and Starlight 1.0.2+.546ae87, the game lags significantly more, going from a low of over 40 and a high at about 87 to a low of about 12 and a high of about 57 (approximations from what I can remember from testing).
After removing each of those three mods individually (and testing on the same world each time), I found Starlight to be the main cause of the FPS decrease. However, my framerate seemed to still be slightly lower, even without Starlight. Although this could be from other factors such as more things in the world having loaded or whatever, I still think it's noteworthy.