On the server, generating mashups should take about as long as loading the underlying number of posts (20-100). On the client they should take that long, plus 1-2 seconds for the AJAX call and Google to draw the map.
It’s hard to tell why a server is slow (mine if often slow without any mashups at all). But if you think the mashups are slowing your site, my first thought would be to increase the memory available to PHP and WordPress (see the FAQ).
The plugin isn’t set up to host maps on another domain. If you want to try it yourself you could create a blog on that domain and copy all the posts with maps there. Then create a template that returns just the post content (no header/footer or sidebars) for mashup posts. Finally, embed links to those posts in an iframe on your main blog.