Voxel learning project performance issues

I currently make a voxel game for training with unity and I am currently on a "major" performance issue: https://puu.sh/D6eEE.mp4

Whenever I move to generate more chunks my FPS go down horribly until the chunks are finished and combined into one mesh, if I generate them inactive and then activate the combined mesh I get a laggspike once I set all the combined meshes active.

Changing from one 1x1x256(height) to 16x1x256 makes it faster but the fps are dragged down far worse.

This would be my code: https://github.com/SidiaDevelopment/VoxelWorld/blob/master/Assets/Scripts/Terrain/VolumeChunk.cs