I have exactly the same issue. And I would say it is a major issue. My environment is: Arch Linux GNOME Shell 3.26.2 geany 1.33 (generated on 2018-03-06 with GTK 3.22.28, GLib 2.54.3)
The issue is present also running `geany -c /nonexistent/folder`. I am encountering exactly the same problems described by @bentref. I can confirm it is not CPU related.
I am using geany since > 5 years and I have never experienced anything similar. If there is anything I can do, I'll help gladly.