Metal Slug Mission 1 - comparison 12MHz vs. 18MHz
On the left running at default 12MHz, on the right at 18MHz. Timer is started on the first non-black frame after pressing start, and ended on the last frame before timer resets for next stage. This way some stage loading time is also included.
12MHz: 1:08.926
18MHz: 1:03.229
Played on actual MVS hardware, so the runs are naturally not 100% identical. Sorry for poor video editing.
Point of this comparison is to give some idea of real-world difference between NeoGeo MVS default clock speed and maximum that Metal Slug can be played at without any glitches. Emulators are capable of removing lag entirely, this is the best real hardware can do.