Here are several benchmarks comparing MTAs:
http://www-dt.e-technik.uni-dortmund.de/~ma/postfix/
a>
http://www-dt.e-technik.uni-dortmund.de/
~ma/postfix/bench2.html
According to these, Sendmail is faster than qmail, exim
faster still, but Postfix is faster than any of them.
It should be noted the authors of the above benchmarks
partake of Postfix-flavored Kool-Aid---YMMV.