To unlock the Speed Champion event you need to collect 11,500 bricks. This is basically every possible brick challenge in the Lego Valley with the exception of the challenge for completing the Speed Champion event.
The event itself is a 2 lap version of the Lego Goliath which is a road race and you need to come 1st. Depending on your skill you can set it to the lowest difficulty and blast through it with no issues.
The only cars you can use are the Lego Speed Champion cars. I personally used an X-class Senna for quickness (it takes about 6-7 mins to do the whole race).
This achievement and the "Eventful" achievement will unlock at the same time and will likely be your last 2 achievements in the DLC.