Back in 2003, when I first made the attempt to offline compress the OS, it was an absolute night-and-day performance difference in startup and daily use, thanks to how crappy hard drives of the day were. I didn't say on the full disk, I just said to use it by default; Microsoft could have improved almost everyone's experience for little effort, even if it was only for the Windows and Program Files folders.
Now I have an SSD, and only enable compression on disk images and the OS to fit a little more until I can upgrade it. Performance difference is pretty much zero, when I've benchmarked, because the overhead of compression was designed to be low for 20-year-old CPUs -- it's undetectable now. (Unless you force LZX mode, which I'm too lazy to.) Sure, the SSD itself would compress for performance purposes, but it won't actually give you back any of that extra space.
For the external mass-storage disk, of course, there's little point in bothering.
The days of resource constraints that can be relieved by workarounds aren't behind us for everyone just yet.