Hi,
Have you tried local playback either on usb or from the sd card?
If it plays fine, I would consider changing to kernel mounts using fstab or autofs. kernal mounts have better throughput than kodi lib mounts.
You could also consider adding a video cache:
https://kodi.wiki/view/HOW-TO:Modify_the_video_cache
Example 3 would be a good start.
Thanks Tom.