drastically different runtimes in BIDS and the SSRS Web Portal
If your query is returning a lot of rows across a slow WAN link then you might be seeing a slowdown due to network traffic.  I have that class of problem where I'm working now.

If you report server is on the same fast LAN as the database server it would be quicker under these circumstances.  This might explain the difference in performance.

Also, if you have virus scanning software running on your PC it could interfere with the local processing in a variety of ways - scanning the .data file etc.  Often real-time scanning also hooks into memory allocation and scans data segments of applications.  This really slows down managed applications.

Those are a couple of possibilities that come to mind.  There may be other causes as well.