Define "slow" in this case. Defining might be easier if there were a video showing the difference.
That said, I checked your config.xml, and you aren't using Crosswalk. Crosswalk provides a Chromium webview for your app, which should improve matters, and provide a consistent environment across all supported Android devices (4.1+). See cordova-plugin-crosswalk-webview-pgb-adapt
My guess is that your device is providing a much older version of the webview to the PhoneGap version of the app, and older webviews have horrible (and often broken) canvas implementations. The only fix is to use Crosswalk or limit installation of the app to devices which have Chrome-based webviews (IIRC, 5.0+).