Thank you for having tried and for your comment. Ehm, what can I say, I know exactly of what you are saying and is definitly an issue I have tried to resolve and I will try again with more time.
Is the clouds BG generation that causes that moment of slowness but strangely enough is also what is making work the part composition of the level at runtime. The idea was to fix the relative position of the clouds making them just above the camera but changing (ore entirely removing) even a bit the first and only clouds generated batch and removing the slowness will break apart the first stacking of the next part of the level. I realized this at hours before delivery when trying to fix the cloud as I had made the one you see at beginning of the dev and put them aside for later. Thank you for pointing that out, I find it interesting and I hope to understand why and fixing but I believe that maybe I will have to find another way to trigger the stacking of the level part each time (that is a big workaround I had to find and I'm not happy about it) but I will need to know more of kaplay to change it. thx again