Just chiming in here really quick, as I can see how frustrated some of you are. Totally understandable.
On the other hand, I see some speculation, so I want to address that with what we know. I'll conclude with some recommendations based on AdMob's stated best practices.
Let's start with what we know:
1. From a technical perspective, there's nothing architectural about Corona that's causing the slowdown.
In particular, AdMob does NOT run inside the Corona "enterFrame", as was mentioned earlier. AdMob runs outside of Corona, and is competing with Corona for system resources, so if AdMob HTML5 ads are eating up CPU resources, there's nothing Corona can do about it.
2. This DOES look like an AdMob issue. It is NOT unique to Corona.
A quick Google search confirms this. The pattern is that some people see this issue, while others don't --- it really depends on the content you're making.
For example, here's one thread discussing the issue (http://www.badlogicgames.com/forum/viewtopic.php?f=11&t=12558) and here's a thread on Cocos2D-x (http://cocos2d-x.org/forums/6/topics/34052). There was even a thread on the Unity forums from a month ago that comes up in the search results, though (oddly) that thread appears to now be deleted...
3. We did look into this issue further to see if there's anything we can do on our side. Unfortunately, we didn't see anything that's within our control to magically make this go away (or else we would have done it by now!)
So here's what we recommend, and some of you probably don't want to hear this, but this recommendation comes straight from AdMob's best practices (https://support.google.com/admob/answer/2790280?hl=en):
Think about the user
Our ads are designed to visually complement the content of your applications. Good ad placement can provide publishers with reliable revenue without sacrificing user experience.
Think about the flow of user engagement within your application. Place your ads where users might be expected to be less engaged with the content of the application, such as transitions in written content, during level changes of a game, or when a user might be expected to exit the application.
It's pretty clear from what Google recommends that they are optimizing AdMob for situations where CPU resources are freely available (e.g. transitional points such as "during level changes of a game"), not for situations where there's competition for them (e.g. during a game level). So based on that, I would recommend rethinking your ad placement strategy. To do otherwise, risks facing an uphill battle against Google!