Terrible news for you, people work on dogshit dells given by their company and netbooks, not high end laptops. Macbook Pros, high end laptops, etc are exceedingly rare. (And can also deal with the high memory usage by virtue of having more memory).
>you want a background picture which fills the entire window
Which you rarely do, and also you don't develop for either, you're going to have a 1080p image at best, and then a bug report from that one client saying "background is blurry" in your backlog for the next 5 years.
>I have mainly used pangocairo, which is based around CPU rendering text to a pixel buffer.
Then you're using it wrong/using the wrong tool. Re-paying text layout at every render is already painful and it should be cached, your font rendering should absolutely be in a texture atlas.
>If DWM has only one buffer which all windows share
Slightly inaccurate in my answer there, but DWM only keeps one final composition buffer, which it handles itself from the various windows (that do have their own buffer, but only for their size and are only kept active if the window is visible.)