I'm preloading ad at the beggining of each layout, but I will add "refresh" every 60 seconds too (but do I have to preload ad after refresh?)
And I have applied for MoPub Marketplace, to get some backfill.
I hope it will helps.
btw. what minimum eCPM had you set up for banners and interstitials in MoPub Marketplace?
I checked Chromium via CocoonJS and it gave me white screen
but anyway it's slower than CocoonJS
so I aprreciate native ads,
but I think that XDK is good only for simple games (because I don't want to see jittering)