Because these are two completely different things.
Testing over lan uses your phone browser and performance varies between devices. On iOs despite a very good browser, test over LAN has some performance issues. But they disappear in a production environment.
Nexus 7 uses Chrome as browser which has a very good and accelerated HTML5 support.
Unfortunately, when you pack your app using a wrapper like phonegap (which is supposed to use the device native browser) your performance will drop since google still keeps their webview bounded to the older, slugghish and bugged android browser with no hw acceleration and a lot of other minor issues even on Jellybean devices (Source: me, today I sweared a lot about this playing with my Jellybean tablet)
CocoonJS is a completely different thing.
CocoonJS will be always faster than the native android WebView (HW acceleration vs SW rendering) but will be always slower than Chrome browser (it's made in google running on a google device so there's an high optmization behind)
Another thing: Avoid very large tiledbackgrounds with CocoonJS.