Originally I was a slow, methodical reader, and still am for my lifestyle/non-fiction books. In terms of fiction, I started slow, but the more I read, the quicker I became. Now I could read a 250 page book in a weekend - it still takes 10-14 hours. Longer books take, er, longer.
Here's a quick link I found with reasons for slow reading, and tips on improving - YMMV though. http://www.reachoutmichigan.org/learn/suggest.html
It's not the speed that counts, its your own comfortable pace. I find it annoying that some people I know could read a whole book an evening, yet the same book would take me 3-4 days. Go at the speed that's best for you, and ignore other people's speed.