Background image on iPhone 4 causes webapp to run slow

I'm porting one of my projects to iPhone 4.

It's a Sencha Touch application, running via PhoneGap wrapper (that means UIWebView). Everything runs rather smoothly on iPhone 3(GS) and the simulator ofc. But on iPhone 4 (we've tested on several phones) the css rule for panel background makes the whole开发者_如何学JAVA app run very slow including scrolling and tab transitions:

background-image: url(../images/background/main_panel__background.png); /*320x317*/

That causes all the stutter and lags.

I've tried to convert the image to JPEG and replace it - still nothing helps.

What could it be? I know that WebKit rendering engine does some scaling (1x1 -> 2x2) to accomodate for high resolution of "Retina" displays, but why would it cause such slowdown?

Thank you.

I discovered the same issue with background image and i am not sure what could be the reason. Actually i suspected the Alpha of my png to cause the issue, but apparently you had it with jpg too... For now i remove the picture. We could use CSS to remove it only for iPhones...