From your posts, internal security doesn't sound like an issue here.
With such a small network, it makes more sense to just set it up as a workgroup.
Seeing that you're behind a router, I'm guessing that takes care of IP assigning so you don't need Windows Server DHCP services.
Obviously as the size of the network grows, having a DC will give you less headaches in the long run.
Without knowing what software you're running, your hardware seems well suited to take on 10 workstations.
Upgrading to a 10/100 switch is your next best move.
A company I worked for had a similar setup except with 25 workstations on a 10/100 hub.
Everything was running fine... the biggest bottleneck was bandwidth.
I was begging them to upgrade to switches.
Oh, and obviously use NFTS as your file system for security purposes (don't rely on share permissions only) and make backups regularly.
Also, to answer your question, I don't think there's going to be a noticeable speed difference between domain or workgroup. For your purposes, speed will be dependent on your hardware.