Very long time to load the map

Can you provide a link to your page with map?

That’s very odd as it should not take so much time only for 20+ addresses. Even so, it should only take longer on the first time the page with map is loaded, as any subsequent page load should be fast because all marker addresses would be already geo validated by Google and cached.

Are you using any caching plugins on your site?

Yeah, you are using W3 Total Cache. Which means you are not enjoying the cached map data from the server side. Every time you load a page it will try to revalidated the addresses.

Can you do the following for me:

1. Deactivate W3 Total Cache for a minute or two
2. Once W3 Total Cache deactivated, load the page with the map three times (on the first time the addresses will be geo validated by Google, on two subsequent times you should see page load speed improvement)
3. Activate W3 Total Cache and try to load page with the map again.