Interesting that you suggest to open Mame before launching BigBox. I have tried that with no success.
one thing I neglected to add which was the issue is that my whole build is on an external hard drive (LB,Mame,…).
I decided to move LB and Mame over to internal storage of my pc and I don’t have the issue. I was monitoring the activity of the external hard drive and noticed when I launched the first game I was at the drives limit, hence the slow start.