Update April 2022: After finding a perfect configuration for flawless scrolling in MAME (see separate post), I update this waitvsync entry.
In the default configuration of MAME, the scrolling of some vertical shooters, such as the famous Xevious, tend to stutter or bleed. Lines to not scroll down smoothly from time to time. Here waitvsync helps a lot - especially if combined with -nothrottle, as explained in the other post.
