How we got a 10x Performance boost at Radio Free Asia

Another hint:
Take a look at your portal_types tool and check, if icon_expr says for some types something like: string:${portal_url}/. What happens with this expression is, that your startpage will be rendered everytime when attempting to get the src for the <img/> tag of the icon. These icons are basically shown multiple times when a logged-in user is browsing. This massively degrades your sites performance!

It might be the case, that you dont use this icon mechanism at all and leave content_icon empty in your types/*.xml. In our setup for instance we use web fonts instead and do not need images. To prevent this problem, refrain from using the deprecrated content_icon as it currently contains a bug (which is gonna be fixed) and use icon_expr instead.