preloading.wtf

doing a big activerecord query with a lot of peripheral objects.

adding ".includes" (preloading) speeds it up dramatically by squashing all the
N+1's.

the page then renders fast and spits this out into the logs:

Completed 200 OK in 504ms (Views: 104.2ms | ActiveRecord: 86.0ms)

however, it is then *** 90 SECONDS *** until the page is actually sent down to
the browser (or curl; we've tested with both).

during which time the ruby process has the CPU pegged at 100%.

if we remove the ".includes" (preloading) it goes back to normal crappy
performance without the minute-and-half chasm of nothingness between page
render and browser delivery.

WTF does the preloading cause that after effect!?

Rails 3.2.12

Ruby 1.9.3 (p286 and p327 tested)