PostgreSQL performance deteriorates over time?

Hello all,

how fast PostgreSQL is supposed to be? Assume a table with
three columns (integer, char, char) and with 100 rows and
an index. Using libpq++ simple UPDATE has gone from under
one millisecond to over 20 milliseconds after a few days of
uptime (mostly idle)? VACUUM ANALYZE didn't help, but
restarting postmaster did. (All this with datasync turned
off with -o -F switches.) What am I doing wrong?

Some info: PostgreSQL 7.1.3, running on Linux 2.2.18 (or
somesuch) on 1GHz PentiumIII with 256MB RAM. PostgreSQL is
compiled from sources, gcc is redhat's 2.96-85. I'm a newbie
when it comes to databases, please tell me if I forgot some
vital information. Also if this is a FAQ or RTFM question,
please do tell where can I find it. I've already read
"3.7) How do I tune the database engine for better
performance?"
And, no I don't think tweaks other than datasync make any
difference, since my program is serialized (only one query
active at a time).

Thanks for any help.

    Taneli