Fragment cache hit is super slow every two times

Hi,

I have a piece of html that I fragment cache. For a very strange reason
every two times I request the page, rendering the fragment cache takes a
lot of time.

Example:

1) I request index page, rendering takes 300ms
2) I request again index page, rendering takes 1300ms
3) I request again index page, rendering takes 300ms
4) and so on

Where could that come from???

I try to do some profiling with script/performance/request, but it seems
broken, because when it has finished making requests, it never renders
the results. In Rails 2.1.1 it worked fine though.

Has anyone encountered this problem?

I had forgotten to use production environment for the profiling, now it
renders the results.