There actually multiple reasons for this. The main ones are:
1. CPU/GPU, like everyone is saying. The iPhone 4S just has more capable hardware. Most Blackberries up to now are running pretty lightweight hardware, slow CPUs, and that means that regardless of how fast the data connection is, the CPU still has to work on processing the data being received and make the webpage look right.
2. Proxy. You r web traffic on a Blakcberry has to go through RIM's network, pass throguh their datacenters, get compressed, and THEN it's sent to your phone. This used to be a GOOD thing, back when networks were 2G, very slow, and a proxy that compresses data before sending it along really sped things up.
But now with 3G/4G, and combined with the fact that web servers are now doing their own low-level compression, we've reached a point where the networks are so fast that adding a whole other path (like RIM does) to move data around only adds latency, and slows things down a bit.
(It alos means you can't do much of anyting with a Blackberry if RIM's servers
go offline for whatever reason.)
Some of this MIGHT change when Blackberry OS 10 comes out hopefully later this year. If RIM lasts that long.