I don’t think the rescaling at render time will itself make a difference to rendering speed, but it does mean that you could resize your stimuli in advance to be that size and THAT will make a big difference.
Note that the key to speed here is not the size of the image in Mb (that’s a matter of how much the codec is compressing your images). The key is the size of the image in pixels because that determines how many values have to be passed to the graphics card by PsychoPy. For example:
- an RGB video in full HD is 3x1920x1080 = 6,220,800 values to manipulate and upload each frame
- an RGB video at 600x400 is only 720,000 values so it’s a lot less work for PsychoPy!