I dont have a measurement, but I know I can read very fast if I want; like several pages a minute if its fairly easy reading. When I took AP History, howeve,r it was so sparse and dry that I had to read each page several times over at a very slow pace.
I dont think I have any tips, but I know that when I read fast, I dont really read every word. I glance at the middle of the sentence, and my eyes take in all the important words on either side, and I just get the general idea of that line. So if a line said "the battle was won, but not without many casualties for both sides." I might look at "not without" and get "battle won not without casualties both sides" It only takes one glance or so for me to extract that; I dont go left to right; just look at the middle of the line.
Also, I dont know if this just me or if everyone does this, but I dont actually read the words. I just look at them, and my brain automatically formulates a mental image of what is happening, without actually imprinting the words themselves on my mind. That only works with accounts of events thogh; with dates and other information like that, I read slower and try to implant the important stuff in my mind somehwere.