Hi,
There should be no impact from having many thousands of posts with a map – if you display the maps one at a time. But if you try to show all of the posts on the same mashup you’ll see a slowdown.
I would say for mashups it’s best to limit the query to a few hundred posts, although it’ll depend on your server, etc. You can avoid this by using the ‘posts_per_page’ argument in the mashup query to set the maximum posts to be read.