Parallelization overhead is the extra cost of dividing, scheduling, coordinating, and recombining work; these costs can make a parallel function slower than its optimized single-threaded version. section 7.2.2
When parallel work loses
A bootstrap confidence-interval function was parallelized with ThreadPoolExecutor across 16 threads, yet execution became considerably slower. Its low 1.5% processor use made it a poor parallelization candidate. section 7.2.2
