Not sure if this info helps any further, but my experience is that this is an intermittent problem, and not just with AdMob.
I only use the Vungle plugin and AdMob.
When the fps drops with Vungle, it certainly does seem to recover a lot quicker than with AdMob (probably 90% of the time, sometimes it hangs around longer). Vungle ads only cause a fps drop about 10% of the time.
I have occasionally seen an AdMob ad, with no drop in fps. This is quite rare though, maybe 5-10% of the time?
Sometimes the fps drop will go away pretty quickly. Sometimes it just hangs around like a rotten smell and I end up quitting the app and re-opening it - though this probably only happens 10% of the time.
My phone is only a couple of months old Galaxy 24 mini - doubt very much it has performance problems as there's hardly anything running on it.
I have also seen an fps drop with no ad in sight, but this probably only happens 1% of the time, there's no doubt in my mind that showing an ad is the major contributing factor of an fps drop.
I have not noticed it on any iOS device but I haven't tested as much there.
This issue is now the only thing stopping me from moving to a permanent freemium model. As with others we'd really appreciate Rob if you could keep us up-to-date, (with an ETA if possible) at least every few days - even if it's only to say "nothing new yet". This bug is a show-stopper.