I'm also using Admob on Android, and it's really laggy especially for the first 10 seconds or so. It seems to stabilise after that initial slowdown period.
I've read on here that disabling text ads is the answer, but I can't say I've noticed any difference.
I'm guessing Admob is doing a lot of stuff in the background on launch, but it's really not a great experience -most people will delete the app in the first few seconds if it is laggy or unresponsive.
@tharek What's the revenue like on Tappx?
Using the XDK 'Android' build option is unusable for games.. far to slow. Hence 'Android Crosswalk' is the best option even with it's many disadvantages. (files size, sound problems, lag etc.)