It takes too long to load site, sometimes about 2-5 mins

You need to use cache. If you are interested, a couple of us are learning together in #2456835: Optimizing cache for Open Atrium

This might also be to do with the server you are running it on more generally (does it meet all the minimum requirements?), and your site configuration. Does a normal Drupal installation run faster?

If you spin up Open Atrium on Pantheon you don't see load times this slow. And even large enterprise sites with thousands of spaces shouldn't take that long.

Perhaps also consider using a another caching tool like Varnish?

Other than investigating the issue candelas suggested, and ensuring your server is optimised, there is not much more we can help with here. A lot of work has gone in to improving the performance of Open Atrium recently.

Poormanscron can cause long load times. Be sure to set up a real server cron to prevent the jobs running on first page loads. https://www.drupal.org/cron