We recently added a second machine to our arcgis server site. We did this mostly for redundancy, rather than performance reasons.
After adding the machine, we've had some issues with it being slow to publish, administer and view services in Manager. Using them from the rest endpoint seems fine.
It's so slow I'm concerned about a revolt, but with less than 5 people doing the things that are slow, I do have a bit of wiggle room.
It was fine before adding the second machine.
10.4.1 - fully patched (including both publishing patches)
2 blade servers - different versions of windows (I know - bad!) I plan to upgrade the older one after summer, if I can get that much time.
1 web adapter in a VM
Config store is on a network file share