IEC functionality is divided into separate functional parts. There are queues between the various IEC parts. Conclusions concerning which functional part is the slowest can be drawn based on in which queue build up occurs.
The maximum length of the queues are not a fixed value, it's just considered to be a value that you should aim for performance reasons.
The queues also display a peak length. This value is often equal to or greater then the maximum value. The reason for this is that if messages arrive in bursts then queues will fill up since the subsequent functionality cannot keep up. This is particularly true if no messages have been processed prior to IEC being started.
The best approach to get reliable information regarding buildups in queues is to monitor that particular page in SAT while IEC is processing a relevant message load.
If a functional part is significantly slower than the others, there are properties that control the maximum number threads that the functional part can use.