Characters from Smallville who have the ability to run at superspeeds, display a speed that is invisible to the human eye but almost aware to those who have the same ability.
Notice that when characters (Clark, Bizarro, etc), when they use their speed, their able to perceive (5 senses in motion) in it.
Characters such as Bart Allen & The Martian Man-Hunter, are able to far surpass this category of speed (notice their speed light streams). Reaching a level of near-lightspeed.
From the beginning of Smallville, Clark was able to catch to the speed of an instant bullet. As time progresses, Clark has become faster to a level of instantaneous speed (without nobody knowing his direction).
- The sound effects and the visual effects are only made for the audience. On the show, characters are totally un-aware of the sound the speed makes, or the blur/stream they leave behind***