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Both rectangles are moving at exactly the same speed.
Based on the flash implementation by Jim Cash: https://scratch.mit.edu/projects/188838060/
Research paper:
https://quote.ucsd.edu/anstislab/files/2012/11/2001-Footsteps-and-inchworms.pdf
Source code
You can find (the full source of) Contrast speed illusion on Shadertoy.