Whilst I do not have an answer for the USB 2.1 aspect of this question, I did find an interesting case about artifacts in under-exposure and over-exposure that may provide useful insights.
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We were able to reproduce this issue in the lab and have notified the RealSense Engineering Team about your findings.
Trying a different frame rate might help. According to our tests, at 30 frames per second, exposure times below 60 μs still look good.
Please let me know if you have any further questions!