I had a similar problem to what you describe, most notably on my Gingerbread test devices.
Here is how I setup my adverts...
The first bit is the same as you do it I expect.
// AdMob Stuff
SetAdMobDetails("ca-app-pub-nnnnnnnnnnnnnnnn/nnnnnnnnnn")
//Create and Position the Advert
CreateAdvert(0, 1, 2, 0)
//Make Advert Invisible
SetAdvertVisible (0)
And then, when everything has finished loading etc. and I'm ready to show the advert.
//Make Advert Visible
SetAdvertVisible (1)
RequestAdvertRefresh()
I think it's the
RequestAdvertRefresh after making it visible that stops the large delay by forcing an ad update. Either way, it seems to have sorted the problem out for me at least
Hope that helps...