One of the things that make Path of Exile more engaging than most other action role-playing games these days is the extra stuff that you can do in it that you can't anywhere else. One of these things is racing, which is something that this game has implemented quite well. Having players start fresh in a new league and have them level up as quickly as they can within a limited amount of time results in competitive play other than just straight-up PvP.
In order to compete in a race, players have to know quite a bit about the game in order to know what to do to level up fast. With that in mind, most players who race are those that have played the game for a significant amount of time. They should know where to go and what to do in sequence to maximize their playing time and optimize every action in the game. This includes what items to get, which areas are best to level up in, and what character build to use, among other things.
But doing well in a race is not just about the character build itself, because that would result in a foregone conclusion if the only factor to a race is playing the best build. Any character class can be used for racing, but it does require a few guidelines to optimize play that is necessary for this kind of event. It requires a bread-and-butter strategy and a set of tried-and-true tactics to do well in a race, and everything else that follows is about fine-tuning in order to win it.
For most people, the easiest character class to play in a race, or in the game for that matter, is the Ranger. Being able to stand back and hit enemies makes for higher survivability for the most part, which is crucial in a race. Other ranged characters are also good for the same reason, although there shouldn't be anything to stop you from playing a melee character if you so wish. Just know your class of choice very well first before getting into a race.
If you do use a Ranger or a character that uses a bow, you should play defensively and do what you can to slow enemies down. Kite them and avoid getting hit, and you can also use traps if possible. This should be the playstyle that you employ for most of the race, which is done while running through the area as quickly as you can. Getting more health, armor, and resists should help as well, so buy some rings and a leather belt to go with the rest of your gear if you can.
In each Act, especially the first one, you may have to skip some quests that either take too much time or require you to take a detour from getting to the end of that Act. Learn which ones you have to either complete or skip in order to optimize your run through the race. Once you have all of this information reviewed, you should be able to do well for yourself in the race.