This is fast:

select ba.name,
penamt.value penamt,
#address_line4.value address_line4
from account a
join customer c on a.customer_id = c.id
join branch br on a.branch_id = br.id
join bank ba on br.bank_id = ba.id
join account_address aa on aa.account_id = a.id
join address ad on aa.address_id = ad.id
join state s on ad.state_id = s.id
join import i on a.import_id = i.id
join import_bundle ib on i.import_bundle_id = ib.id
join (select * from unused where heading_label = 'PENAMT') penamt ON penamt.account_id = a.id
#join (select * from unused where heading_label = 'Address Line 4') address_line4 ON address_line4.account_id = a.id
where i.active=1

And this is fast:

select ba.name,
#penamt.value penamt,
address_line4.value address_line4
from account a
join customer c on a.customer_id = c.id
join branch br on a.branch_id = br.id
join bank ba on br.bank_id = ba.id
join account_address aa on aa.account_id = a.id
join address ad on aa.address_id = ad.id
join state s on ad.state_id = s.id
join import i on a.import_id = i.id
join import_bundle ib on i.import_bundle_id = ib.id
#join (select * from unused where heading_label = 'PENAMT') penamt ON penamt.account_id = a.id
join (select * from unused where heading_label = 'Address Line 4') address_line4 ON address_line4.account_id = a.id
where i.active=1

But this is slow:

select ba.name,
penamt.value penamt,
address_line4.value address_line4
from account a
join customer c on a.customer_id = c.id
join branch br on a.branch_id = br.id
join bank ba on br.bank_id = ba.id
join account_address aa on aa.account_id = a.id
join address ad on aa.address_id = ad.id
join state s on ad.state_id = s.id
join import i on a.import_id = i.id
join import_bundle ib on i.import_bundle_id = ib.id
join (select * from unused where heading_label = 'PENAMT') penamt ON penamt.account_id = a.id
join (select * from unused where heading_label = 'Address Line 4') address_line4 ON address_line4.account_id = a.id
where i.active=1

Why is it fast when I include just one of the two subqueries but slow when I include both? I would think it should be twice as slow when I include both, but it takes a really long time. On on MySQL.