CaptureScreenShot too slow on iPad3, any way to capture a lower res for higher speed?

I want to allow the user to take a photo using the device camera but with rendered unity objects overlay on top, then shows users a preview and give them an option to post on facebook or email out, picture will be save on album. So I am using Application.CaptureScreenshot, and then load the captured photo into a texture for preview.

The above logic/code works fine. However, since the CaptureScreenshot takes around 3 seconds to run on iPad3, my photo preview wouldn't show at least 3~4s after the user pressed the photo taking button, so the user experience is not good at all.

Anyone got any suggestions on how to code this better? I am okay with a lower res photo since I expect the user to only use the photo for email/fb. Or can I use the CaptureScreenshot in background, but quickly generate a preview using some other method?

Many thanks