They aren't faster than normal mutexes. They're disabled by default for years, so de facto it's dead code, never used.
Bug #37703 MySQL performance with and without Fast Mutexes using Sysbench Workload
Bug #72805 mutex_delay() creating excess memory traffic, GCC mem barrier needed
Bug #72806 mutex_delay() missing x86 pause instruction optimization
Bug #72807 Set thread priority in my_pthread_fastmutex_lock