VACUUM and performance after pg_dumpall

what is it about pg_dumpall that necessitates an immediate VACUUM to 
restore performance?

in upgrading recently from a pre-7.1 release to a 7.1.3 release for a 
fairly sizeable database, i noticed a severe decrease in performance 
that seemed to be related only to the new version of postgres that was 
running. everything else, including DBI, DBD::Pg, etc., remained the same.

i had done a pg_dumpall to preserve the data.

after a VACUUM, performance returned to within acceptable limits.

does pg_dumpall and the subsequent restoration of data create a bunch of 
proverbial dust that needs to be vacuumed?

-tfo