As a data point, Carver (in Optimal Rate of Reading Prose. Ronald P. Carver. Reading Research Quarterly, Vol. 18, No. 1, 56-88. Autumn, 1982. 56) quoted 200ms/word for optimal comprehension. That's 300wpm.
For more background and references, see:
http://www.psych.utoronto.ca/~muter/pmuter1.htm
Lex