I am quite new to the world of BlackBerry's so my input here should not be construed as the Gospel! But here's what I've found on my phone.
I am the only BB user in our very small company. And while we are small, we do have our own Exchange server with BPS server now running on top of it so that I can access our corporate email, utilizing the 1 free license that comes with BPS. The server is rather old and a bit slow by today's standards. Our office Internet connection bandwidth is a mere 1.5Mbps on both the transmit and receive(full duplex). While this doesn't sound like much compared to what others are able to achieve with Broadband cable or DSL, it suits us very well and we are transferring a boatload of images throughout the day.
Since I am connected through "Enterprise Activation", any requests through the BB browser go from my device, to my server over our office connection, then out to the site I'm trying to get to. The returned HTML pages then come back through to the server and then out to my device. This whole process in inherently slow. I suppose if I had a super fast server and a 10 Mbps Internet link at the office things would be different.
When I tested my Storm at dslreports.com using the BB browser, the best I achieved was 350 kbps throughput. Doing the same exact test with the Opera Mini browser achieved 3600 kbps. Over TEN times faster as it does not get routed through the Exchange/BPS server. It goes directly to the intended site. I get better Internet throughput on my wireless cellphone than I do with our companies "hard" link.
You may want to see if you might be running into some of the same issues I have found...