Cognos Server Optimization

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Hi experts,

I would like to inquire for some information regarding Cognos optimization.
Do you have any documents pertaining to how the Java heap size are utilized?
We have a client that reported their cognos is slow. But upon checking their environment, there is nothing that we could see that can contribute to the slowness of their server.
Currently they have two BI Server connecting to one content manager. Both servers has 8gb heap size allocated, Server 1 have 8processors, server 2 has 4 processors.

Is there a bestpractice guide into how we can improve the overall performance of Cognos? The servers still has unallocated memory that ready if increase in java heap size be needed. But we would like to make sure first on how we can set the proper size or what are the things to consider in doing so.

Thanks in advance!