JUST NOT ADDING UP

 hi all,
something is not adding up. the following query is taking a long time
to run.(its still running right now)

select distinct on (prem) prem, num, addy, mynum,myad,ff.address, matchtype,
the_geom
from daily.recheck2, _sp_myparcels ff
where
prem not in (
select  distinct on (prem) prem from daily.recheck2 dr, _sp_myparcels ff
where ff.address = unabrev
)
AND btrim(addy) = btrim(myad)
AND  num = mynum

UNION

select  distinct on (prem) prem, num, addy, mynum,myad,dr.unabrev,
matchtype,  ff.the_geom
from daily.recheck2 dr, _sp_myparcels ff--, feeder_polygon
where ff.address = unabrev

the thing is if i run the first query by itself, it takes like about 2
seconds, and if i run the subquery that takes about 2 seconds also, so
why (well its now finished, took all of 3.31 minutes) does it take so
long?