Performance with multiple users during transformation

Hi,
I have a 250 KB XML to be transformed into another XMl
document. I am using a high end UNIX box with 8 CPUs
and 32 Gig RAM. 
Intially I was using Xalan and the timings for
transformation are as follows:
1 user : 3 secs
2 users: 6 secs
4 users: 8 secs

Then I changed to Saxon8 and the timings are
1 user : 1.5 secs
2 users: 3 secs
4 users: 4 secs
(I fear whats going to happen when 1000 users will use
the applicaiton)

Although I got a 50% performance enhancement with
Saxon for each scenario, but I still dont understand
one thing why do we have the increase in
transformation timing as the users increase.
Since Saxon is based on SAX parsing there should be no
memory constrained (Also my 32 Gig RAM is never
utilised more than 5GB at any time.)

Can somebody briefly tell me how the transformation
works, starting from how the XSL is parserd, how the
XML is parsed and then the transformation.
Can the parser and the XSLT processors be different.

I fear that the parsing of the XML is done by some
other parser (I use Oracle parser by default) and then
the transformation is done by Saxon, as I just replace
the Saxon.jar and Xalan.jar for testing the 2
processors.

thanks in advance,
Bhupendra.

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