performance problem aftrer update from 7.1 to 7.4.2

Hello everyone,

i just upgraded my old postgres-database from version 7.1 to 7.4.2.
i dumped out my 7.1 database (with pg_dump from 7.1) as an sql-file with
copy-commands and to one file using insert-statements.

after initalizing and starting postgres 7.4 on a different port and
datadirectory, i tried to import the sql-dump with the copy statements.
this import fails, but importing the dump-file with inserts took a long
time but was successfully.
ok, at first i do a vacuum and analyze and fire up the database again.
now i changed my php-scripts to use the new database as datasource.

ok, things look good, but as i was testing some php-sciripts, i
recognized that the querys took about 2 or 5 times longer :(
a test a script which reads a lot of stuff from database, normaly with
pgsql 7.1 it tooks about 4 seconds to display the data, but with
postgres 7.4 it tooks about 25 seconds.
i start the 7.4 pgsql the same way as 7.1 with postmaster -D
/xxx/xxx/...  -N512 -S -F -B2048 -i
i also tried to kill some indexes and recreate them, but this doesnt
matter and the query took the same long time to execute. i also tested
some simple select querys, the will take longer time.

if somebody could help me, and hints on how i can speed up my 'new'
databse would be very nice :)

thanks in advance

yours
volker