Clip freeze at start

For the story: For a theater company, I am moving a show from QLab/MadMapper to Millumin Demo, to see which one we'll use for next shows.

My problem is: when I launch a clip it freeze around 0.5 seconds on the first frame then the clip runs normally. I've tested several file size, from 100MB to 2GB, and with optimized or not videos but the problem is still there. 

Any idea about this problem?

Thanks