You have to think in terms of bottleneck. If you have a really heavy desktop environment or operating system, then it can (and will) slow down older and weak computers. For those, it makes sense to install some special prepared environments, so it does not slow them down. If you have a modern and fast computer with plenty of resources, then it won’t make a difference which you install.

In example, you have 16gb RAM, but your system uses only 4gb. Switching to a system that uses only 2gb won’t get you any benefit, you have plenty of room that is unused. And for all other daily operations in the Window environment, lets say opening and closing windows with some effects and transparency, would lets say for fun require 1ghz of CPU to calculate without slowing the operation down. If you have a modern multicore CPU with 5ghz, then you don’t win anything by installing a desktop environment or operating system that makes use of only 0.5ghz.