index on timestamp performance

I have this schema:

motid | integer | not null
objid | integer | not null
date | timestamp without time zone | not null
Indexes: dico_frs_motid_date btree (motid, date)
dico_frs_objid btree (objid)

The performance I'm getting from the index that contains
'date' is much slower than when using the objid index
(different queries of course). This is a 10 million row
table. Am I right to assume that postgres needs to do
more work because it has to convert the dates to some
internal (integer?) format?
Consequently would I gain performance if I changed the
schema so that 'date' is an integer (which would not
be a problem for my application) ?

Thanks,
--
Eric Cholet