It can be a lot of things. But yea, the xport loop of turboSST is a bit of a CPU hog for older systems.
You need to get a bandwidth monitor like Freemeter:
http://sourceforge.net/projects/freemeter/And run it to measure your bandwidth usage. Then use a test service to find what your output is.
While you're doing that, keep an eye on CPU load. If you're running XP there's a free one built into the control panel (administrative tools -> performance).
You can also just run w/ the task manager open and watch the CPU and ram usage. The goal is to keep your ram usage below your physical ram level, otherwise you will end up task swapping and slowing down.
Do all of that while people run around w/ scripts. Watch what apps spike, how long, etc. It could be anything. It could be a lack of ram, could be you're running on too little CPU, could be bandwidth, could be a firewall that's screwing up, could be a memory leak, whatever. Watch what happens and use that data to improve performance.