A few questions for you.

• Have you monitored both the client and the server during execution?  Does the client churn?  If so, then it could be that the browser is having issues trying to render the html.
• When you hav a 6k+ line report, does it take long to go from page to page? 
• Have you changed any of your logging settings (in RS) to make them more verbose so that you my see something in them.
• I've not looked but I would assume the most verbase (probably called trace) level of logging within RS should show you when the query has been executed, then when the rendering of is starting and ending.  This could be a good place to get some baselines.
• What's your DB allocation like for the RS temp db?  If it's not large (large enough to handle your data volumes) then each time you run these long reports it may be having to allocate further disk to the files which will slow the inserts into the temp tabes, and hen slow the rendering back to you. 
• Further to point above, I thought I saw something (new setting) in SP2 that let's you 'cache' to disk (ie in filesystem) rather than DB, this may be worth looking into in case the overhead of writing to file system is less than th eoverhead of writing to the DB.
• Does your IIS have html compression turned on?  If so, this oculd make things slower (at both ends) as it has to compress, and the client decompresses, the html before rednering can occur.
• Nothing showing (error wise or otherwise) in your event logs?  Nothing in the system log, maybe showing that the system is trying to gain authentication but needs to make multiple attempts to get it fromt he domain controller?

I would definitely recommend trying to break this into sections, and then reviewing/baselining each with logs (where possible).  So look at the SQL execute time (for you running the query in QA but also put tracing on to monitor the same query when executed via RS).  Look at the SQL status (file sizes etc) looking for possible synamic expansion when running reports (in particular the RS temp DB).  Lastly look at the RS logs to see if you can identify any longer running pieces of the execution task.

Lastly, although a report may be only 10 rows, via a browser (even over the intranet at 100/1000 Mb) I wouldn't expect it to take < 1 sec to render (under 2s, yes probably).  I doubt IE can render the html involved that fast.  If you're comparing QA to RS, it's an unfair comparison as QA just presents the values.  Although i've not seen under the covers, I'd be assuming the overhead in what gets sent back to the client (Query analyser in this case) is little more than the actual bytes required to display the characters.