These four squares appear to move at different speeds, stopping and moving, waiting for each other in pairs. In reality, they are always moving at exactly the same constant speed. Seriously. I opened the GIF in Photoshop and used guides to measure it -- in every frame, the squares align perfectly.
Their motion is exactly the same but the stark contrast of the lines and the light yellow and the dark blue makes them look desynchronized while in motion. Your brain has difficulty seeing the yellow edges against the white and the dark blue against the black.
The yellow appears to speed slow down as it enters the white lines, while the blue appears to slow down as it enters the black lines. In reality, it's all an optical illusion. Only when the squares get close to each other your brain realises they are moving at the same speed.