If you use a magnetic ball with a non-ferromagnetic roller coaster such as aluminum, you can slow the ball down a lot. https://www.youtube.com/watch?v=c_Wu3dizBKc
But be warned, event officials might think the magnetic ball is not allowed because the rules say the roller coaster cannot have magnets. However, in my opinion, the ball is not part of the roller coaster (the rules make a distinction between "roller coaster" and "vehicle"). Also, a magnetic ball will not be attracted to a non-ferromagnetic material like aluminum when not moving. It's only when it moves that the ball feels a retarding magnetic force because of dynamically induced electric currents in the aluminum as the ball rolls down.
In my opinion, this is using science
to come up with a better engineering solution, which is what Science Olympiad should encourage. Unfortunately, you will likely get tiered with a construction violation if you do this. I have submitted a FAQ question months ago on this, but no response.