Studies have shown it looks faster and in UX perception is everything 😉
A study (PDF) demonstrates that animations can increase the perceived speed of a download by up to 11% over a bar that is not animated. Having a reverse-animated background as in the Gmail loading bar, or having the background pulse faster as the bar nears completion, both create this effect. There is a video demonstrating the animations and summarizing the article.
 Harrison, C., Yeo, Z., & Hudson, S.E. (2010). Faster progress
bars: Manipulating perceived duration with visual augmentation.
Proceedings of Computer-Human Interaction.
This took forever to make using Image Ready. lol
When the spiral is going against the bar direction, it does visually exagerate the speed of the bar movement.
The bars are pretty close to each other, so hide one with your palm and look only one at time. 🙂
This is an assumption, but it’s likely because it makes the progress bar appear to fill more rapidly.
This effect is achieved because the right edge of the bar, the consequential part, is moving in the opposite direction from the animation, thereby making the increments that bar edge moves appear larger than they do relative to the box containing the filling bar.