What is the fastest homebuilt experimental aircraft?

As of my last update, the fastest homebuilt experimental aircraft is the Q1, designed by experimental aircraft designer and pilot, Brian L. McCoy. The Q1 achieved speeds exceeding 400 knots (about 460 mph) in level flight. This aircraft showcases advanced materials and aerodynamics, pushing the boundaries of homebuilt aviation. However, advancements in aircraft design and technology may lead to new contenders in the future.