1) Use Windows Fast User Switching. This is a good idea. Right now we use 1 user account on all machines and it's always logged on. So this would be a change for people to get used it, but might be worth doing.
Do you have AD or P2P?
2) Upgrade to gigibit. I think if I got the Gbit router only the new main machine and the NAS drives would use it. I'm pretty sure the 5 year old laptops don't have it, but maybe the next rev.
As long as the server has it, that should be good. Think about it. Your server can now supply those laptops with their full 100 mbit bandwith. Gigabit can give 10 laptops the max bandwith they support where as with a 100 mbit, you have the 100 mbit shared between all devices.
3) Defrag, haven't done it but the machine is pretty new, but worth a try.
You should definitely do that then. Especially if it's on 24/7, the hard drives will get very fragmented. SSDs don't need to be defragged which is another reason why I like to use them. Don't really need any maintenance.
4) SSD, well we just got the new main machine with regular drives, RAID 1. I certainly wouldn't want to switch unless I was sure that would fix the problem. The way disk caching by the OS works pretty much everything should be sitting in RAM. We have 24GB of RAM. So a completely fresh start after booting the server might be slow. But for the testing I was logging in/out dozens of times, at that point everything should be in RAM that can possibly be in RAM. I can run tools to see exactly how many bytes are being moved around on the main machine when one of these re-login happens. Does anyway have an apples-to-apples AC-specific benchmark of HDD vs. SSD? Would be interesting to see.
That's not correct. SQL Express will only cache a max of 1GB in memory. As time goes on, the extra pages go to the page file goes to the hard disk which may possible make it even slower. That's why a lot of people see a speed boost after a reboot.
I should probably record a video. I installed several computers in an office. The doctor bought his own computer from Dell with an HDD. He wouldn't stop complaining that AC was slow on his computer after using mine in the exam rooms (with SSDs). He actually started doing all of his work in the exam rooms. Then I upgraded him to an SSD and he's back in his office. For some people, it's a big difference. Ever since I got my first SSD, I ended up getting them for the rest as well. I now don't have a single computer without one.