SSRS 2005 Subreporting Performance Concern

Question

I have a report which, when rendered on its own, has the following performance times (taken from ExecutionLogStorage table):

TimeDataRetrieval:  6776
TimeProcessing:  142
TimeRendering:  30

When this report is used as a sub-report which is repeated 34 times, the performance of the overall report comes out as follows:

TimeDataRetrieval:  9255
TimeProcessing:  187709
TimeRendering:  35

Furthermore, the memory consumption of my IIS process (using ReportViewer web control) goes up by several hundreds of MB.

Are these performance issues inherent to sub-reporting or is there something wrong with my report?