I reported this same issue to Apple Accessibilitty Support and I got this response as well
I've reported this to the appropriate group but please do go ahead and file a webkit bug.
I can confirm that same behavior is on Iphone 6 and Ipad Air 2 running latest version of Ios 8.4.
I can also confirm that that bug isnt happening on Ipad 4 running Ios 7.
Bug 1 ::
If you click on a element on a web page that has onclick event then Voice over stops responding or is really slow to respond.
For example if we have button on a web page defined like this.
Bug 2 ::
Also if a web page uses window.alert Voice over stops respoonding or is really slow.
I created a test page so you can check yourself.
STEPS TO REPRODUCE ERROR OR BEHAVIOR
a) Turn on Voice Over
b) Visit http://tv.titlovi.com/apple-test/bug1.html
c) Click on a button "Test button".
This issue is fixed in 9.0. You'll be able to test with the public beta. I don't have easy access to the duplicate bug number right now