I recently saw a very similar problem - it started a few months ago, through several constant reports and seemed to be caused by hard or soft page breaks (I found this from a lengthy troubleshooting process).
This scenario was SQL Server Reporting Services 2012 SP1 through the Native / Report Manager portal.
Does your report have page breaks?
My solution was to set Report / InteractiveSize / Height to 1000 cm. Then for each hard page break I turned it off to render the browser by setting the Page Break / Disable property using this expression:
=Globals!RenderFormat.IsInteractive
The result is a bit untidy in the browser, but displays page breaks in other formats (PDF, Word, Excel). Importantly, he stopped browser freeze in IE.
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