Thanks all *relief*
beufreecasse I had very poor performance to begin with on my 820. If you upload the entire Windows Phone project to a remote location, then change the following in MainPage.xaml.cs:
private string MainUri = "/index.html";
to
private string MainUri = "http://[your remote location]/index.html";
and
Browser.Navigate(new Uri(MainUri, UriKind.Relative));
to
Browser.Navigate(new Uri(MainUri, UriKind.Absolute));
Test out this and see if the performance is better. If it is then there's a way to vastly improve performance.