I did an other test and I confirm the switch point for USB2.0 is at 5 seconds and working! Below a screenshot of the gradient at 5 seconds using USB2.0. It is severe, most likely while USB2.0 is not working at optimum speed. With this gradient imaging is not workable. Some pixels are 1.6 seconds longer exposed!
Assuming a gradient of 1% is acceptable, the switch point for USB2.0 should be at 106 seconds. The amp glow is not a problem since dark subtraction will solve it. At 4.99 seconds the gradient by amp glow is at value 378 against an average pixel value of 308, so still minor.
The switch point will be always debatable and depending on the setup of the user. So I think you should allow the user to select one of a number of fixed values E.g. 1, 2, 5, 10, 15, 20, 30 (50, 100?) seconds. Default at 2 seconds. See below a proposed modified menu:
menu proposal.png [ 32.54 KiB | Viewed 151 times ]
cls 5.0 sec, binx4.jpg [ 93.86 KiB | Viewed 151 times ]