What sort of internet connection do you have to serve 600 people? you may need to consider to bandwidth shaping / QOS to get the best out of your connection.
If your max children limit is being reached, then you need to push it further, otherwise requests to filter the data will be queued until another process becomes free.
When you say slows down, what exactly do you mean...sites start to timeout? or network traffic to ClearOS slows down?
How much RAM do you have in your server?
If you disable the content filter (but keep the proxy with authentication on) does network speed resume normal speed at peak time? If so would suggest a server resources problem in filtering all the traffic
The top output at peak time would be useful if you can post it