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. With the help of some forum entries, I finally found a very decent way how to get rid of it without side effects.
Step 1: waitvsync - 1
In QMC2: Tools/Options/Emulator/Video/waitvsync - 1 (enabled)
In my configuration, this immedately smoothens the video output, perfect vertical scrolling. However, the audio output suddenly gets some hickups. Some forums explain this effect and it seems that sound samples get out of sync.
Step 2: speed - 0,99 (Xevious only!)
In QMC2, choose Xevious from the games list and toggle the machine specific configuration:
Here I reduced the game-specific speed from 1,00 to 0,99 - et voila, the audio stuttering was gone.
A hint from a M.A.M.E forum, many thanks for this! Xevious now scrolls perfectly and has decent audio output.
Update: After some testing, I tend to activate waitvsync only individually for games that need it. For games without bleeding, it seems to me that deactivating waitvsync provides better results. Games in which I currently activate waitvsync individually are:
1942 (no speed adjustment required)
Cheeky Mouse (no speed adjustment required)
Elevator Action (no speed adjustment required)
Phoenix (Speed 0,98)
Scramble (no speed adjustment jrequired)
Shinobi (no speed adjustment required)
Wonderboy in Monster Land (Speed 0,99)
Xevious (Speed 0,99)