Use zopfli with 1000 iterations, pngout /r with various blocksizes and deflopt.

assets/ico/apple-touch-icon-114-precomposed.png | Bin 2939 -> 2931 bytes
assets/ico/apple-touch-icon-144-precomposed.png | Bin 3614 -> 3549 bytes
assets/ico/apple-touch-icon-57-precomposed.png  | Bin 1577 -> 1573 bytes
assets/ico/favicon.png                          | Bin 889 -> 887 bytes

Total difference: -79 bytes.