I will post here the reply from the support ticket you opened on the same question:
"In general, report processing and rendering are memory intensive operations. During the report generation, besides the retrieved data there are newly generated resources by the report which are held in memory. Thus, the machine should have enough memory and CPU to handle the amount of data and the document processing.
For more detailed information and suggestions for improving the report's performance please check Performance Considerations help article.
Note, that controlling when the garbage collector fires is not done by us. You can force the garbage collection if required - check this MSDN article."
In the end of this reply, there is a list of the additional information we would need in order to provide you further suggestions.
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