How to detect Bottleneck

Hi All,
I have following query.
I had 2 app servers.
Load avg for one of the app server was 1.5-1.75 and memory used was around 50% but the run length queue was around 12. What could be the possible reason for this?
Load avg for other app server was 5-5.75 and memory used was around 95% but run length queue was between 3-4.
I am not able to get to a conclusion that why app server with lesser load avg had higher run length queue.
Please Help
Regards,
Gurpreet