Frametime spike when killing :
The game ( freezes / lags / stutters ) when killing a zombie.
There is a big frametime spike, displaying a stutter clearly visible when using RivaTuner ( framerate / frametime monitoring tool)
The lag spike is felt normally during gameplay as well, however:
The freeze is easy to identify by doing a circular motion with the mouse while killing zombies. (without 3rd party tools)
Video Demonstration: