I have the same stuttering problem, I can share my particular subtitle, if it will help you diagnose the problem.
I have observed that the longer the text, the more frames are skipped, the longer the video stops. The shorter the text, the less the effect is pronounced.
I have determined that the old beta version of VLC did not have the problem, playing the same subtitle.
I have also tested the stock Android video player, smooth as a lightsaber through stone.
MX Player Pro > smooth as New Zealand butter.
But I prefer the way VLC treats subtitles, slightly over everyone else. (Still not as perfect as PC's version of MPC-HC.)
I'd like to see more srt flexibility, being able to place text in X Y positions, colours, bold, italics, 2 texts at once (overlapping with no problems).
My subtitle has these aspects to it, and if Videolan implemented this, they would be the only ones who provide this flexibility on Android.
Let me know if you could use the srt