I don't know about lag, but my planets and moons wont even appear on the screen :/. I don't know if this is spam or something, but this is the second comment I've said about it and I already posted something about it so yeah :/
I use 5G RAM, my pc has 10G so I use half the amount of the G my PC has, galacticraft 1.11.2-126.96.36.199, another mod installed = optifine
I tried playing on 1.12.2 and I got a warning that "Planet, bla bla bla from galacticraft core etc" were missing, like a heck tone of things were missing in 1.12.2, idk. but that's all. I downloaded optifine because of this insane lag
I tried removing half the amount of lights/solar pannels/ satelite dishes and display screens, killed all mobs around removed some itemframes, removed ALL nasa benches, removed half amounts of oxygen collectors (I use them on rooftops as those generators for air conditioning, just aesthetic) and removed all hanging structures I can find laggy enough, ex. : vines. Maybe the rooftops cause lag, I am not sure. My base is pretty detailed. I'll remove some more stuff now.
I get alot of memory garbage collection and tick lag, and fun fact, gratings cause insane amounts of lag! I replaced them a while ago with iron trap doors.
I'll give a list of stuff that I believe might cause lag:
-launch controllers with battery inside them
-fuel loaders with or without battery inside them
-open hatches or doors in buildings (one of them which started lagging had 2)
-Lighting pannels : linear and square
-item frames (all gone now)
-fuel liquid/asteroid fluid
-quite alot of lava under the map (in doubt about this)
-heavy aluminium wires
-tin decoration block
-plant pots/flowers (in doubt again)
-forests and trees planted by me(in doubt again)
-CHUNKS BEING LOADED.
if your using optifine, which i avoid to no end. What graphics card do you have and cpu. Reason why is while minecraft is very CPU heavy the use of optifine and shaders uses a gpu (if one is installed, if not then integrated graphics), the cpu will always become the bottleneck for anything graphics related, which can be mistaken for game "lag" when its really graphics lag.
based on your list of stuff i suspect your "lag" to be caused by optifine
-CHUNKS BEING LOADED. < this will always have an effect on actual game TPS, however if your in single player or the only person in the world this will not be an issue (unless your using a massive amount of chunk loaders is various areas where your player is not currently located)
EDIT : There was no difference at all between minecraft with optifine and minecraft without optifine. My game crashed again. The only difference is:
With optifine : More memory is used, somehow.
FPS reached out to 400, but when Iook at what I built, 73 to 60 to 0
Without optifine : LESS memory used
Fps reached maximum 270, but when I looked at what I built, 73 to 60 to 0
I believe it's the memory that chunks use whenever the map is loaded in.