Subject Local loopback vs. gigabit ethernet
Author Ann W. Harrison
This is from a report Jim made on tests with Nimbus... don't know
if it applies to Firebird applications, but it is interesting and
unexpected, at least to me.

To make a long story short, the problem was a change in
the way I ran the tests. If I put the load program on
the same node as the SQL node, a dbt2 load takes about
260 seconds. If I run the load program on a different
node, the time drops to 77 seconds. The load program is
doing approximately nothing. The difference is relative
performance between local loop back (bad) and bona fide
Gigabit Ethernet (good). An astonishing difference.