Comment 6 for bug 1311873

I looked into this more, and (at least on my OSX machine) the couple seconds of lag is due to drawing all of the font previews for the combo box entries (https://github.com/PintaProject/Pinta/blob/7dcd1794470e985fd3a1f937ffbea46406988e7e/Pinta.Core/Extensions/ToolBarFontComboBox.cs#L54).
GTK seems to be triggering this before the combo box is actually opened up, and is trying to render the preview for every single font.

I'm not sure there's too much we can directly do - we might need to consider alternate approaches for the UI (e.g. use https://developer.gnome.org/gtk3/stable/GtkFontButton.html to instead launch the standard font selection dialog). Or maybe this performance issue will be fixed entirely in GTK3...