Win10 v1909 b18363.997
See if this happens on a current version of Excel. Mine is ancient (Excel 2003) and I couldn't possibly expect it to be addressed. It's a little obscure, and if it happens on more current versions it could be more relevant, or perhaps provide insight to some "Groupy causes high CPU usage" problem.
To test, have Groupy turned on and Excel opened with only one file open. Hide the file/workbook and see if each the Excel and Explorer processes throttle. (Command to hide an open workbook on my ancient Excel is Window/Hide, hopefully it's something similar on a current version.)
Happens while Groupy is merely turned "On", doesn't need to have anything grouped. If Excel is open specifically with only a hidden window (hidden file) open and with no visible windows open, cpu usage for Excel and Explorer will each throttle. Unhiding the hidden file, or opening another file. or creating a new file, or closing Excel, or turning Groupy "Off" will stop the throttling. (If you turn Groupy back "On" while Excel remains in the same state, the throttling will start again.)
An example of where this would tend come into play is a worksheet of macros to be used across multiple Excel files, saved as "Hidden" both to protect it and because you don't really want it in the way.
I thought I'd pass it along in case it's useful. Thanks.