Are maps the only thing slow to load? Is your Tableau Server running in "offline" mode? I suspect the "slow load time" is actually the delay while Tableau Server is timing out after trying to access our mapping servers. It will then display the offline maps when it cannot hit the online maps. If this is the case, you can tell workbooks to automatically use the offline maps instead of trying to load the online maps first. Check out the "Offline Maps" section of this article: Select Background Maps
If the Tableau Server is not supposed to be in offline mode, check out this article to verify proxy and firewall settings: Maps Do Not Display on Tableau Server | Tableau Software
Hope this helps!