Slow movie startup WD Mycloud Mirror + Shield + SMB2/SMB3

Plex Server Version#:

Plex Server is on Shield Pro
movies are on WD My Cloud Mirror NAS, latest firmware, connected vis Gigabit LAN

When NAS is set to SMB1, all movies start instantly. When switched to SMB3 or SMB2 it takes 15-45 sec to start a movie.

Please, help