"Show CPU Usage" not an option Nexus 5X, running build number NMF26F, rooted.  "Show CPU Usage" is not an option available in Developer options.
Unfortunately the current solution (running 7.1.1 build NMF26U on Google Pixel) was to completely remove the option, as the feature was broken in 7.0

https://code.google.com/p/android/issues/detail?id=213571