I don't think the problem with the more hefty window managers is that they use a lot of RAM, but that they have to use a lot of RAM. IMO, the fact that they re

I don't think the problem with the more hefty window managers is that they use a lot of RAM, but that they have to use a lot of RAM. IMO, the fact that they require more memory can be considered a reasonable indicator that they are more complex / do more stuff. They need to read stuff from disk / generate data in order to fill that memory in the first place, which takes time. I'm not an expert, but I would guess that using more memory also increases the chance that some of it will be swapped to disk by the OS, slowing things down further (although only intermittently).