SELECT performance problem

I am updating from 7.4.5 to 8.2.3.  I have noticed a significant
slowdown in simple searches such as
     "select filename from vnmr_data where seqfil = 'sems';"
This returns 12 rows out of 1 million items in the table.
On 7.4.5, this takes about 1.5 seconds.  On 8.2.3, it is taking
about 9 seconds.

I have played with different values of:
work_mem, temp_buffers, shared_buffers and effective_cache_size
and none of them make any difference.

I am running on redhat Linux 4   64bit.

Any ideas?

Glenn