big preformance drop

every time the game start, after a few seconds there is a big gc collect and a few frame rates drop that lasts over a second, after that every thing works smoothly, only happnes when unity ads is integrated.

any way to solve it? checked every sdk version from 3.1.1 to 3.4.2 on unity 2018.4.15f1
need to mention there are over 6 ad placements configures in the dashboard