Stutter in video when first subtitle is displayed




I'm running 2 rasplex 1.8 (latest version) on 2 Raspberry Pi 3 B connected via LAN.
I experience a little video stutter (0.5 sec) whenever a subtitle is displayed for the first time or when there is big delay between the last and the next subtitle.
I tried all kind of options in rasplex, like sync playback to display on or off, A/V Sync method tried all, resolution and refresh rate, hardware acceleration on or off, force transcoding on or off, always transcode subtitle off and on, changed buffer to 1% and 30% it makes no difference.
Same problem on 2 pi's, one in livingroom and one in bedroom.
I don't have this problem on Plex Media Player for embedded devices on same 2 pi's.
But I like Rasplex more because of more expert options to set (crop upper black bar, display subs in lower black bar, audio video and subtitle offset while playing movie).
How to debug? what else can I try?