Video Choppy When Using Sup Sub File

[KurtHorne]

I have several .mkv videos with soft subs. The videos play just fine except when I load the sub file (.sup). When the subtitle file is loaded, the video pauses for a few seconds at frequent intervals. I've tried several of the videos and sub files in the series I have and all present the same issue. Not sure if anyone knows what's going on. Running the latest version of VLC (3.0.12) Windows 10, loads of processor capacity.

Any ideas appreciated. Thanks.