It could be the kind of cell you have, what have you built? In my experience the sprinting does increase the speed for reasonably sized cells (so like size 10 cells get a very noticeable speed increase while sprinting if using the full sprinting bar).

As a sanity check I just tried with a single hex cell and while the initial sprint kicking in isn't super noticeable the speed once the sprint bar is full has increased quite a bit. The sprinting is balanced to really just either give you the chance to eat something or escape a predator. So it is meant to have a more reasonable increase to speed that helps a bit in those kinds of close call situations.

This is no longer the case but the AI used to not be able to sprint so if the effect was big it would have given a totally massive advantage to the player. So it is balanced to give a noticeable speed increase but not massive. Even now I think our microbe AI could probably be made to sprint a bit more intelligently.