The slider changes the amount of memory and CPU resources that can be allocated to certain services. For example, maximum queue sizes, maximum number serviced per second, etc.
There were many individual options that used to cover these things.
The slider bar is a way to configure all of these settings more easily. Each notch in the bar represents a set of settings applied to a variety of options. When you slide to the right, all the maximum queue sizes go up, the maximum serviced goes up, etc.
In general, the slider controls how many Extreme Management Center server resources we dedicate to processing End-System events. These resources may generalize into:
CPU: More CPU cycles are dedicated to processing End-System Events which increases the event processing rate.
Memory: The size of queues in memory increase allowing Extreme Management Center to buffer more events before events are dropped which helps with bursty events.