Ok, that explains what I see in the profile. The profile groups things by overall times, so editing functions take up most of the time, and whatever the scrolling contribution was, it was swamped by the noise, and doesn't really stand out.
To attack it from the other end, have you run with the QT_X11_NO_NATIVE_MENUBAR environment variable set to see if that makes scrolling hitches go away? It should certainly make the terminal messages go away, but I'm not sure if it's related to the performance problem?
I need to run that version of SlickEdit on a Ubuntu 16.04 and see if I can at least reproduce the messages and see what's causing those.