Why didn't my gigabit network card result in better performance?

It depends on what it was you were expecting to change.

I ran into a very similar situation when I upgraded one of my switches to
gigabit and saw only a marginal performance impact. So I started looking into
why.

First, let’s make sure you’ve actually covered all the bases.

In order for a gigabit (or 1,000 megabit) connection to happen, the
equipment at both ends of the wire need to be gigabit-capable. You indicated
that you purchased a gigabit ethernet card, but what’s it plugged into? If it’s
a router or other device that’s only capable of 100 megabits, then the
connection will happen at the slower speed. You can’t force a gigabit into a
100 megabit connection.

Similarly, if you’re copying files between two machines, then both machines
must have gigabit-capable network connections for you to be able to take
advantage of the higher speed.

And if you’re expecting your gigabit network card to somehow improve your
internet speed, then you’re going to be very disappointed. Your
internet connection already has a maximum speed – a cable connection might
occasionally go as fast as 10, or even 50 megabits, but that’s slower than the
card you already had! Putting in a faster network card will not speed up the
internet connection provided by your ISP.

So far I hope all that makes sense.

Now, let me tell you about a scenario that isn’t nearly as clear, and had me
puzzling over it myself for a while.

My desktop machine came with a gigabit ethernet adapter. I decided to speed
up the connection between this machine and a backup machine by installing a
gigabit ethernet card in that machine and connecting the two through a gigabit
switch.

Now, even though a gigabit per second is 10 times faster than 100 megabits
per second, I didn’t really expect a 10x increase in speed, there are too many
other factors at play. What I didn’t expect was that the increase would be more
like 2x. My ten-times-faster gigabit connection was copying files
machine-to-machine only twice as fast as my 100 megabit connection had.

Clearly the gigabit connection had some effect, since there was a
speed up, but it wasn’t what I was expecting.

My assumption was that there was another choke point in the system. Just
like the gigabit connection to your ISPs internet connection will never be any
faster than whatever the ISP gives you, there must be something else in my
system that was slower than my newly speed gigabit connection.

And indeed there was. I did some tests and discovered that the gigabit
connection was faster than the transfer rate of my hard drives. The hard drives
had become the slowest component in that machine-to-machine transfer. While the
network was capable of faster speeds it was being held back by the work
required to actually get the data on and off my hard drives.

Does this mean that the gigabit network is pointless? Hardly.

While in the past that machine-to-machine copy might have actually saturated
my network, since the 100 megabit network was at that point slower than the
hard drives and hence the limiting factor for the copy, now the copy could
proceed at the hard drive’s full speed, and still leave lots of network
bandwidth for other traffic. Other machines and other network activities could
continue without being nearly as impacted by something that would previously
have used up all available bandwidth.

In practice, do you care? Probably not. At least not yet. As always, speeds
and needs will increase so that eventually that 100megabit network will seem
slower than molasses compared to future technologies.

But for now, for most home and even small business needs, a 100 megabit per
second network is plenty.

And upgrading may not even get you the increase you were expecting.