Create a comp group with many resources, go to the monitor tab. See it take some seconds to compute the page. Then on the top in the summary section, click on the right arrow to see the next 5 resources of the group. Depending on the display interval (8h, 24h) this will take more time on a larger interval (e.g. for 180 Session Beans it takes 2s for 60 mins and up 50s for 24h). Looking at the Oracle Top activity output indicates that this may come from the fact that each time, the measurement graphs are re-computed -- even if the results of this recomputation is not shown on screen.
Actually, after some more thinking: If the graphs would update on the cycling through, this would still be slow, but 'correct', so I am not sure if it is a correctness or performance issue.