The existing multipart sender used to send the world (planet and voxels) to clients sends one chunk at a time, waiting for the full round trip ack before the next chunk gets sent.
Because the existing chunk size is 64kiB, and considering an effective 200ms rtt (which isn't unreasonable considering simulation time and physical network latency), this comes out to 320kiB/sec (2.5MiB/sec) max.

Steam's maximum chunk size for reliable packets is 1MB, so we should be able to safely increase this chunk size to 512kiB, which should make joining large multiplayer worlds significantly faster. The possible downside is if the host's upload link gets saturated the large block size makes it more likely already joined players will be affected.