Running performance test for WP ticket #17019 to show:

cost of superfluous call_user_func_array() calls in apply_filters_ref_array()
cost of context check to avoid them

Premise:

M filters are added to 'post_where'
but each filter only applies to only 1 of the N queries, and will return an unaltered WHERE clause for all other queries.

Results:

Queries	Filters	superfluous call_user_func() (current)	additional empty() (proposed)
1	0	0 msec	0.02 msec
5	0	0 msec	0.01 msec
2	5	0.06 msec	0.01 msec
5	5	0.14 msec	0.01 msec
5	10	0.29 msec	0.01 msec
5	20	0.6 msec	0.01 msec
10	5	0.25 msec	0.02 msec
10	10	0.57 msec	0.03 msec
10	20	1.2 msec	0.02 msec
10	50	3.09 msec	0.02 msec
20	50	6.16 msec	0.05 msec
100	100	62.43 msec	0.25 msec

test done.