Why is restored database faster?

I backed up my database using pg_dump, and then restored it onto a different
server using psql.  I see that the query "SELECT COUNT(*) FROM myTable"
executes immediately on the new server but takes several seconds on the old
one.  (The servers are identical.)

What could account for this difference?  Clustering?  How can I get the
original server to perform as well as the new one?

Thanks.
-David