Postgres is too slow?

I've created the same table with 0.5M records ~200 bytes each. And I
decided to compare its speed under Postgres 7.1.2, Linux (kernel 2.4.1) and undex MS
SQL 7.0 (SP3), Win2K Server on the same PC - PIII/500 MHz, 128 Mb RAM.
Under W2K the OS stealt about 64Mb, and Linux eated almost
nothing, cause XWindows doesnt work. The speed of data exchange with
HDD was almost the same under Linux and Win2K.

Under Postgres I filled this table by COPY FROM cause.

And I made the SELECT that browse the whole table:
-----
select field1, sum(field2-field3)/count(*)
from table1
group by field1;
-----
1) It's speed almost doesnt depend from index on field1.
2) I made VACUUM ANALYZE

And ... MSSQL 7.0 worked in 2-2.5 times faster that Postgres :((

Why so?

-- 
Best regards,
 Paul                          mailto:magamos@mail.ru