There are a lot of factors.
If you can close out firewalls, proxyservers, intrusion preventions, you can try an alternative Distribution an Patch Setting on the DMZ Server.
Choose a setting that does not throttle cpu or bandwith.
What version are you on? There are some known issues with CSA download times on 2017 versions. This was addressed in 2017.1 Service Update 1. See the readme below.
We are on 2016.3 of Management Suite.
We have this working now.
We found that the preferred server was causing the problems.
We only had one listed on the Core which pointed to our DFS share name which obviously worked fine on the LAN.
In the DMZ however the DFS share name could not be resolved so we added another Preferred Server entry with the fqdn of the core server and when the PreferredServer.dat file was then generated on the clients in the DMZ they could resolve the Core server name properly and download speeds were just as they are on the LAN.
Took some getting there but we did it in the end.