This inst really noticeable but it is for a sure a thing.

While on a render distance of 6-8, You barley notice but it sills lowers FPS.

If you put it higher you see a massive FPS dip while looking down, Now i know this isnt a huge concern but something im guessing underground is causing some FPS issues.

Yeah, I noticed that too. My guess is that it's the mob spawners, will have to do some testing though.

Could be possible, The entity limit seems low thou.

My second guess would be the spike things in the cavers, as iknow 1.10 rendering can be a laggy mess xD

Nah I don't mean the mobs, I mean the spawner block.
I doubt it's the stalactites because those should be culled away and not rendered at all unless you're in the caves.
There was a bug in previous Forge versions where TileEntities wouldn't be culled, but that was fixed recently so make sure you're using the latest version of Forge and then check again.

Im using the latest version of the mod and forge :)

Closing. A year old.

This is what the underground looks like in spectator mode in 2.0.4-alpha. (Unless I'm messing things up with some other mod, which is totally possible.)

It's like that pretty much everywhere.

If I stand at the threshold of stuff-get-culled, you can see that MC culls it all,

but sometimes fails.

In comparison, here's some vanilla cavegen:

Changing the chunk glDraw call to use GL_LINES should make it obvious if things not getting culled is a problem. You might be able to help it with the culling by dropping the caverns down 16 blocks so that they're at least a chunk-layer below the surface. This should make it super quick & easy for MC to cull. Then you'd just need to restore accessibility to the player by some means.