I can finish a 500 page book in about 4-8 hours. Building the scenes in my head is much more of a bottleneck than the actual mechanics of reading. Complicated scenes, e.g. The Pillars of the Earth, which is heavy on architectural terminology I'm not familiar with, and a huge amount of characters, e.g. Good Omens, slow me down considerably.

Also, being within earshot of people having a conversation slows my reading down to a grinding halt so my technique to read faster, outside of reading more, is basically finding a quiet place.