I did a quick test on my Mac using 4 threads instead. At "heavy contention" the spin lock is actually 22% faster than parking_lot::Mutex. At "extreme contention", the spin lock is 22% slower than parking_lot::Mutex.

Heavy contention run:

  $ cargo run --release 4 64 10000 100
      Finished release [optimized] target(s) in 0.01s
      Running `target/release/lock-bench 4 64 10000 100`
  Options {
      n_threads: 4,
      n_locks: 64,
      n_ops: 10000,
      n_rounds: 100,
  }

  std::sync::Mutex     avg 2.822382ms   min 1.459601ms   max 3.342966ms  
  parking_lot::Mutex   avg 1.070323ms   min 760.52µs     max 1.212874ms  
  spin::Mutex          avg 879.457µs    min 681.836µs    max 990.38µs    
  AmdSpinlock          avg 915.096µs    min 445.494µs    max 1.003548ms  

  std::sync::Mutex     avg 2.832905ms   min 2.227285ms   max 3.46791ms   
  parking_lot::Mutex   avg 1.059368ms   min 507.346µs    max 1.263203ms  
  spin::Mutex          avg 873.197µs    min 432.016µs    max 1.062487ms  
  AmdSpinlock          avg 916.393µs    min 568.889µs    max 1.024317ms  

  $ cargo run --release 4 2 10000 100
      Finished release [optimized] target(s) in 0.01s
      Running `target/release/lock-bench 4 2 10000 100`
  Options {
      n_threads: 4,
      n_locks: 2,
      n_ops: 10000,
      n_rounds: 100,
  }

  std::sync::Mutex     avg 4.552701ms   min 2.699316ms   max 5.42634ms   
  parking_lot::Mutex   avg 2.802124ms   min 1.398002ms   max 4.798426ms  
  spin::Mutex          avg 3.596568ms   min 1.66903ms    max 4.290803ms  
  AmdSpinlock          avg 3.470115ms   min 1.707714ms   max 4.118536ms  

  std::sync::Mutex     avg 4.486896ms   min 2.536907ms   max 5.821404ms  
  parking_lot::Mutex   avg 2.712171ms   min 1.508037ms   max 5.44592ms   
  spin::Mutex          avg 3.563192ms   min 1.700003ms   max 4.264851ms  
  AmdSpinlock          avg 3.643592ms   min 2.208522ms   max 4.856297ms