just ran into this problem and I think it is quite an important bug and most likely easy to fix.

USECASE:

1) Apply Billboard (Facing Camera) to scene
2) Apply any popVideo that has transparency to Billboard
3) Press Play -> Frame Rate is 60 fps
4) Now go in the timeline and move start of video to Frame 1000
5) Return to frame zero and Press Play -> Frame rate and performance drops drastically between frame zero and frame 1000

WORKAROUND: Create empty transparent video between frame zero and frame 1000.

Please avize.

Thanks.