I'm finding "gm compare" to be faster.

    $ time ./imgdiff cypress.png cypress.png         
    Success! Images are equal.
    3.22user 0.10system 0:00.50elapsed 661%CPU 
    (0avgtext+0avgdata 478524maxresident)k
    0inputs+0outputs (0major+48339minor)pagefaults 0swaps
    
    $ time gm compare -highlight-style assign -highlight-color purple -file diff.png cypress.png cypress.png                                             
    1.99user 0.21system 0:02.21elapsed 99%CPU 
    (0avgtext+0avgdata 874948maxresident)k
    0inputs+5240outputs (0major+217173minor)pagefaults 0swaps

    $ time gm compare -metric mse cypress.png cypress.png                                                                                                
    Image Difference (MeanSquaredError):
               Normalized    Absolute
              ============  ==========
         Red: 0.0000000000        0.0
       Green: 0.0000000000        0.0
        Blue: 0.0000000000        0.0
     Opacity: 0.0000000000        0.0
       Total: 0.0000000000        0.0
    0.94user 0.14system 0:00.67elapsed 161%CPU 
    (0avgtext+0avgdata 585832maxresident)k
    0inputs+0outputs (0major+144881minor)pagefaults 0swaps