This needs to be reopened.
In GZDoom 4.5.0, enabling mouse smoothing in options doesn't make a difference anymore.
The recent NightmareLogic by Dr_Cosmobyte exhibits this problem as well, so I'll use it to showcase the problem.
It's only 811 KB in size, all you have to do is download the mod, type IDFA and select the plasma gun.
It uses SPF_INTERPOLATE in its Ready state.
Lower i_timescale to make it more noticeable.
Mod's @
viewtopic.php?f=43&t=70984Recorded footage for your convenience.
I only recorded @ 30 FPS, so in reality it's actually much worse.