The MongoDB team implemented a hybrid approach to reduce queue times for code changes by parallelizing testing among multiple machines while keeping key parts serial. This allowed them to achieve a 62% reduction in wait time, from an average of 1,238 seconds with the serial approach to 469 seconds with the hybrid approach. The team introduced speculative execution to ensure that submissions run tests with the latest code changes and maintained an ordering concept to prevent unnecessary work. By balancing parallelization with serial parts, they were able to reap the benefits of both approaches while meeting the requirements of fast performance for typical sizes of the problem and handling worst-case scenarios gracefully.