I think removing all of those things won't really help with the lag situation a lot. It might seem like a lot of blocks but that doesn't effect the performance that much. And while the lobbies are usually massive, the game maps are normally much smaller and way easier to run. I'd say the bigger problem are all of the animations and custom models, that are being loaded at the same time and there are quite a lot of those in lobbies and even in games.
At the end of the day, there is also another important note to remember with all of the mentioned buildings and cosmetics. There are at the end of the day what makes CubeCraft the server that it is, it gives it that unique and personal identity and is what makes Cube different from other servers. And it's really hard to get rid of that, especially knowing that all of those cosmetics are the biggest revenue stream for the server.
There are also probably other elements that effect your performance meaning deleting all of those things might not even help with your problems. And as far as I know Cube is also working and making sure the performance is the best as it can be with everything that is being loaded, so I'd take that as a positive as well. I know it might not be the answer that you were looking for, but this is my outlook on the situation. I hope the things get better for your performance.
