Accuracy of psychopy keystroke duration measurement
In the Psychopy forum I have received some advice:


One would be to change the code: take out the print command, no
redefining of variables 
USB, USB device, the python and system
libraries should be the newest and fastest, if possible, because
these could have an influence on the accuracy. 
And that Psychtoolbox forum could have the solution, but so far I haven't found it.