tfouto I have a Nvidia Shield, currently running 9.1.1.
The OS itself causes absolutely no strain. However the same cannot be said for certain streaming apps.
The Netflix, AMC+, and Disney+ apps I have installed, currently cause no strain in the slightest.
The same used to be true for the Amazon Prime Video and Paramount apps. However after both apps forced me to update them (in order to view content), the Prime app now causes almost immediate strain after just a couple of minutes. And though the Paramount app is nowhere quite as bad, now seems to be causing some minor strain.
So unless there is a feature you would like out of the latest OS, I would caution against it -- as I'm not sure if the streaming apps themselves look at your current OS version in then forcing you to update the app version itself(?). But then again, as long as the current OS is good, it may make sense to update it now, while you can.