I ran into a bug with GTK2 stalling out with some malformed DBUS messages (symptom: your application takes an additional 20 seconds to start). So I made a sample app that demonstrated the problem, ran some system traces, tried to engage the GTK developer community and only got an "eww, gross, not touching that".
It's pretty clear that GTK2 has been left out to bit rot at this point and it's going to completely collapse at some point. Only option is to update your old apps.
Thanks for the link, I had not found that myself and the workaround suggested (installing the Unity integration module for GTK) worked! It generates a spurious error message about Unity not being on the machine, but that's way less annoying than a 25 second pause on startup.
It's pretty clear that GTK2 has been left out to bit rot at this point and it's going to completely collapse at some point. Only option is to update your old apps.