I wonder that too. Been doing some research, I read somewhere on the misc UI optimizaitons forums that with Screen Space the UI moves with the camera, and since the UI is moving, it has to keep rebuilding itself since it’s now ‘dirty’. I would really like for that to not be so true. Others have experienced no performance difference switching between the two so it might be dependant on your seupt, no definite answer so far.
[EDIT]: Moving the canvas itself doesn’t cause a rebuild, but moving one of the elements in it does according to Unity UI performance optimization tips
Maybe if somebody from the UI team could chime in on this to confirm, would be great!
[EDIT] Actually Phil from Unity talks about this here UI Canvas related questions
Quote just in case the link gets lost
“Its also a performance thing. When the canvas is Screen Space - Camera or World Space the UI geometry get put into the main render queue to be picked up by the camera rendering path. (i.e. it does culling check, sorting, ect) where Screen Space - Overlay gets draw directly to screen so it avoids all the extra stuff the main render queue does.
That being said the overall performance difference should not be noticeable.”
[Edti]: Compiled a list of performance tips from my findings Unity UI Performance tips - Sharing my findings