Performance decrease

I'm new to PG and I'm testing default PG settings
for now.

I have PG 8.1.3. installed with autovacuum=on.

My test table has 15830 records with 190 fields.
I have different fields types (date, numeric, varchar,
integer, smallint,...).

I decided to evaluate PG because I need to use schemas.

First test I did is not very promising.

I tried to update one fields in test table several times
to see how PG react on this.

I do like this:

update table
set field = null

After first execute I get time 3 seconds. Then I repeat
this update. After each update time increase. I get
4 sec, 7 sec, 10 sec, 12 sec, 15 sec, 18 sec, 21 sec.

Is this normal (default) behaviour or I must do something
to prevent this.

Regards,
Radovan Antloga