Steps to reproduce:
Create a site.
Select the attached space as content source.
Enter theme editor
Preview the content source overview page in 'detail' or 'hero' layout
Theme editor immediately previews the space as it's being edited.
Preview is loading for a very long time.
The main reason for the slowness is that the whole page tree will be built also for the preview where it cannot be used (and where all the pages would not otherwise be needed like when generating the whole site).