When on the 'Resources' tab of gnome-system-monitor cpu usage climbs to 90-100%. Come off this back to the processes tab and it comes down to a more acceptable 3% or so.

Try to change Resources -> Graphs -> update interval to anything other that the current 0 secs and the setting is simply not remembered.

Is this a bug in CentOS 7, or does it affect other distros as well?

I've seen GUI based apps which are written incorrect end causing 100% usage because while they give up the processor, they have zero timeouts to keep them being responsive or have them emulate real time reporting. The end result is that while this doesn't kill the system or slow it down, it is a loaded situation which can cause stability problems.

When you run the monitor, does it slow the system so that you almost can't use it, or is the system still normal and just the CPU usage statistic is bad?

My read if the system operates near normal is that the monitor application would be the problem due to poor coding.