Raising this on behalf of Adobe. Page rendering takes a huge performance/lag hit for pages within large spaces that make use of the pagetree and children macros. This also affects large spaces using the Documentation theme. The slowness is directly attributed to the way both macros decide which pages should be rendered in the list which is visible to the current user.

Given that this particular space has 11000 pages, the pagetree macro takes around 2 minutes to render depending on where you are starting within the tree.