Coalesce function in where clause

Query with coalesce function takes less time than other queries. below is the one select query which takes 20k timerons

select * FROM tablea a
WHERE SOUNDEX( a.lastname) = SOUNDEX('pim') AND SOUNDEX( firstname) = SOUNDEX('tim')

when I add some meaning less coalesce function with 'where ' clause, it is taking less time. below is the select query with coalesce function which takes 14k timerons.

select * FROM tablea a
WHERE SOUNDEX( a.lastname) = SOUNDEX('pim') AND SOUNDEX( firstname) = SOUNDEX('tim')
and a.lastname= coalesce(a.lastname, a.lastname)

Can somebody explain how the above query taking less time than the first one. appreciate your help.