Stutter in android related to admob plugin

Very strange and annoying problem with my latest game. I’m getting significant stutter in gameplay when admob data returns (so when an interstitual ad loads) - this is me pre-loading ads before the next display.

Even stranger is I have 2 games - both I have just done updates for. both pre-load ads while game is playing. my newest game has always had this stutter, but my old game I updated has no stutter…can’t figure it out.

I can’t see any difference in the games, indeed both use the same code for ads.

I have removed everything from my new game while still leaving it playable - almost all graphics and overlays…and still it stutters a lot when the ad data returns - also for 5 secs or so after, and then it calms down to smooth gameplay.

This is on device - android - not in the simulator - no ads in sim.

also I should note the stutter is in all transitions, not just in gameplay responding to player input - if I just leave the game alone I see stutter in the background animated elements. (the enemy sharks which circle - this is in my released game “monkey diver” on google play now.

more info - I changed to not pre-loading ads. now the stutter is there and happens after an advert is closed.

why?!?!