Hello,
I would place the diode at the position where the stimulus appears.
Do you need the high refresh rate? Otherwise reduce it to see whether the run becomes more stable.
Did you turn off unneeded services such as network, virus scanner aso.?
Look here Preloading all stimuli - #9 by Michael and here
https://groups.google.com/g/psychopy-users/c/fWSl36UFYUA (ignore the last message with win.flip() This is only relevant when you are using the Coder instead of the Builder.
Best wishes Jens