Version 4 of Default Folder X (and currently version 4.0.2) re-introduced a nasty bug that persisted for a long time in earlier versions but which was finally squashed within the last year. But now it is back. According to Activity Monitor (already on your computer, so you can check this for yourself), the %CPU usage starts at 0% when Default Folder X starts then gradually (but quickly) rises to several percent.l It is not uncommon to see CPU usage fluctuate between 5% and 11%, even as the program is inactive. I don't know if it goes higher if left alone.