Slow to open WebView when changed User Agent on Android

I changed an user agent in my Alloy controller. So it's very slow to open a webview and a display is blacked out.

controller.js

$.webview.userAgent += 'MyAppName/' + Ti.App.version

webview.xml

<Alloy>
    <Window>
        <WebView id="webview" />
    </Window>
</Alloy>

If I comment out the code to set a user agent, open a webview quickly. I tested my app on some Android 4.X devices, but every devices same. What can I do to fix this?