A report migrated from 2005 but since redeployed from BIDS 2008 R2 had InteractiveHeight=0. In the example it returns roughly 5000 rows. 2005 rendered this in about 30 seconds (subjective, not measured, can't re-stage environment), independent of browser. The SQL behind the report runs <5 seconds, the report has no sorting or groups, but a lot of dynamic columns, which justifies the horrible rendering time.
Under 2008 R2, this runs over 9 minutes under IE8 using 100% of one core. Interestingly, using the x64 build of Namoroka (aka Firefox, version 3.6.3, newer x64 builds are 3.7 alpha only) is way faster - 2 minutes, however, only after continuing after the "hung script" error message. Namoroka stops spinning the "Loading" gif after 40 seconds, displays the initially visible rows after 75 seconds, and is done after 155 seconds.
Removing the InteractiveHeight element from the RDL solves the ""problem"" by breaking the report into ~90 pages, the first of which appears after about 20 seconds in ie (12 in Namoroka) plus 5(4) for each "next page" click. Thus, the total time is the same but the patience requirements become bearable.