AlterBrains

Hi, I want so implement a site with this structure:

• mainsite.com
  content1.mainsite.com: displays some specific content
  content2.mainsite.com: displays some other specific content

users and authentications should be shared
user will have different permissions on different sites (ie can post on content site but not on mainsite)
content, menus and templates will be different

I'm not sure whether I should keep component separate or not

Which configuration would you suggest with mightysites?

The main site is using Community Builder extensions

Please check last two case studies, you can have single Joomla installation:
https://alterbrains.com/joomla-extensio ... se-studies

Content will be same but displayed differently per site via site-specific menu items.

Thanks, I'm considering the last one

The final result should be similar to the joomla.org portal, with a main menu always available to browse through subsites, and submenus for each subsite

Will it be possible to share just the main menu through the websites?

Also, contents on some of the subsite may increase considerably, should I expect performance issues on all the portal if using a single installation?

Only all menus and menu items can be shared.

Also, contents on some of the subsite may increase considerably, should I expect performance issues on all the portal if using a single installation?

Sure, it's same as running multiple sites.