As of Jul 5 2013, when we visit the page of each filter, e.g.
there is a line with statistics like this:
Of the last 4,255 actions, this filter has matched 7 (0.16%). On average, its run time is 3.55 ms, and it consumes 1 condition of the condition limit.
(nowadays, the last information is unavailable unless $wgAbuseFilterProfile = true)
There should be a special page which displays a table with the data of the above line for every existing filter. See an example at ptwiki:
This makes it easier to identify filters which may be improved to reduce the number of conditions or its execution time.