Yes, there is nothing using CPU, just firefox with 1%.
Do you mean even Geany isn't using CPU?
Anyway, one possible candidate would be the new-ish accessibility support, could you try re-building Geany without it, like commenting out/removing [line 3070 of *scintilla/gtk/ScintillaGTK.cxx*](https://github.com/geany/geany/blob/master/scintilla/gtk/ScintillaGTK.cxx#L3... (not actually tested but it should work)