Since MAME 0.226(?) most Popeye versions now stutter unbearably. These are: "popeye", "popeyeb3", "popeyef", "popeyeu", "popeyehs". Additionally "popeye" also plays parts of the demo melody way too fast.
Please fix this. Some bootlegs still run perfectly (on Win10, Intel Core2Duo@1600MHz).
All of those sets sound fine on my 2020 Core i7 MacBook Pro. If audio is stuttering you're probably not running at 100% speed.
R. Belmont is correct.
My checking indicates that the audio stream refactoring/rewrites likely caused a ~40-50% drop in Popeye's performance compared to other recent versions, first observed in MAME 0.225 and based on a small benchmark test (-bench 20). Machines with such dire CPUs as posted (posted cpu tech appears to be nearly 15 years old) are more likely to see the speed issues if performance dips.
As far as it being faster at times during demonstration, normally throttled emulation sounds fine to me. I'd suggest checking your speed (F11) and use (F10) to make sure the throttling is enabled.