I don't have a server, but the problem you're facing sounds a lot like a problem with the server memory. If the server host has an option for this, you'll probably want to set it to as high as possible for the best performance, although that can cost money. If the problem persists with max memory you can try to pre-generate the chunks, but I don't know how to do that.

pre-generation of blocks is simple, I'm more afraid that there will be some error that the server will not fully count on. I also counted individual chunks and if I pre-generated the whole LOTR world it could go up to 70GB and unfortunately I have only 50GB to use: D

If you can you can always select which areas to load, so that it won't take up all the space.