How do I stop the Windows 10 Photos app (and Runtime Broker) from pegging my cpu?

One way that might or might not work is to deactivate it from running in the background. Press Win + I, then go to Privacy, then to Background Apps. Switch off Photos.

Or, if you want to be a bit more drastic, you could simply remove the photo-app - this works and does not harm the system (though of course, you would need another viewer/organiser for your pictures).

Just fire up a PowerShell-Terminal as Administrator and type:

Get-AppxPackage -AllUsers "*Photos*" | Remove-AppxPackage -Verbose

_{Derived from my very own remove_win10_apps.ps1.}

Further reading: Remove appx package for all users