From what i understand, it's optimum value is 2. I personally have set mine to 20 as i have a lot of OnPCLoginEvent scripts which would ordinarily exceed the queue value.
There's no magic settings to "optimize performance". The more scripts you run on a server, the more resources it will use. The more players you have connected, the more scripts you have running.
The best answer i can give you for number 3 though, is to avoid scripts that use timers/loops. Scripts that use static data perform better than scripts that use dynamic data. Keep your variable usage to a minimum by using bitmasked values. Event scripts that spawn x amount of monsters in a specific location at x times of the day are discouraged, as that will create your resource bottlenecks.. but if you know where they are, then you can do things to combat them