We periodically experience just the type of slowing you describe. I discovered the following to be the cause:
The Updox connector runs on our server. It should use about 30-40mb of memory (per Updox support). Periodically, that number rises. We have seen it above 600mb at times. When it gets above 200 or so, all the computers using AC slow down.
I know this makes no sense; when I first noted it, no one in our office or at Updox could understand this, nor could they understand why it should slow AC. Everyone is now finally convinced because the problem is recurrent, and fixing it immediately fixes the slowing.
You can reboot the server, but it is much quicker to go to the Task Manager, stop the Updox connector process, and then restart it. This only takes a few seconds, and can be done while everyone else is still working. The increase in speed is instant.
I showed this to Tobin a year or so ago. I think I finally convinced him that it is real, and the source of our slowing. On the other hand, he had not seen it before, and while he offered to work on it, did not see it as a priority since no one else has reported it. We live with it because we only see things slow every few days, and can fix it in a few seconds.
So...the next time your system slows down, check the memory use of the Updox connector, and if it is high, restart it and see if it speeds things up. Please let me know one way or the other; I would like to get Tobin working on fixing it! Some people believe that there is no such thing as a "memory leak" but that is how I would describe this.
