alancoelho
An update of software for my NVIDIA K2000M graphics card enables the card's so called threaded optimization. The effect this has, when an OpenGL window has been opened, is to slow down TOPAS non-OpenGL processing by as much as a factor of 3.
The slow down continues even after closing the OpenGL window; only a restart of TOPAS fixes the problem until the next OpenGL window is opened.
Disabling threaded optimization from the NVIDIA control panel fixes the problem.