When the map starts some mice can't move for about 0.3-0.5~ sec. That's enough for others to get first.

Edit: not only racing has this bug.

That happens to me alot too.. but I think it's all because I lag. I'm not sure :p

It's a lag and everyone or the other with slower internet has it.

This was a bug, not lag.
It has been fixed according to players in this thread: Topic-623737

The knew way the countdown works is irritating as hell, and as a result you usually lose a few seconds to other players as a result, as the OP said.

The way it worked previously where you ran before the countdown was up was fine, not sure why it was changed yet again.