when X page (a problem page) loads, what exact times and % of the total load time is taking place at:
http server
ror, broken down into: activerecord, non-AR logic, & rendering
web browser (pulling down JavaScript files, CSS files, etc)
(calculated for several runs, with perhaps an offpeak benchmark & onpeak benchmark to see if there's a major difference)
Chances are, 80% of your slowness can be alleviated by a few small changes in one of the above areas, but each app is different of course.