Limitations in input field value size in form post? Not OK to post to iFrame? (Safari on OSX10, browser crashes)

I am doing a form post where a hidden input field's value is set to a base64 image from a HTML canvas. This can be quite large.

In most browsers (including Safari on Windows) this goes through fine but the performance is quite slow in Safari on OSX. And worst of all, if the image approaches 800x600 and has decent variation (large size) the browser becomes so slow it becomes unresponsive and actually crashes.

Another thing I am doing is setting the target of this form POST to an iframe on the page, so the page itself does not unload (I handle the loading indication in the UI myself).

With the post to the iFrame (on a different domain) I get errors in the Developer console, but apparently these are not a problem as other 3rd party services from Google and Facebook also have this issue. (see http://stackoverflow.com/questions/3010170/unsafe-javascript-attempt-to-access-f rame-with-url-error-being-continuously). I mention that because maybe it contributes to the slow performance.

Any info on which of these things is causing a crash/performance problem would be greatly appreciated

Thanks!

Chris