I don’t think this is related to bug#18407; I don’t suffer from slow-downs. Emacs instantly freezes after using the “eval line and step” icon from the tool-bar in QUICK succession. I believe it is time-dependent meaning if I use the tool bar slowly no lock-ups occur. Also  C-g does not unfreeze emacs. I use Force Quite (Yosemite OS x) to terminate emacs. I also believe that the tool-bar is directly connected with freeze.