So, that shouldn't be taxing your memory too much.
I couldn't reproduce your problem. I ran your program verbatim on my windows system running Vista64, (AS)5.10.1 and IO::Compress:2.060 and on files 65/210MB and 80/600MB and they both took roughly 5 seconds to disk and 3 seconds in memory.
Does the same problem exhibit on all files when decompressed to memory or is it confined to one particular file?
One possibility (mentioned by Corion) is indicate by excessive page faults for the process(*). If the process shows a page fault delta greater than double digits per second, you have probably encountered the malloc problem. But if that were the case, I would have expected to be able to reproduce it here on my standard AS install.
(*You'll need Process Explorer or work out how to use perfmon.exe to find this information.)
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I've updated the original post to have profile data between a good run and bad run - maybe that will help.
It seems to be dawning on me, that the problem has more to do with the old AS 5.8.9 that I'm using rather than with the IO::Uncompress::Gunzip module. However updating will take lots of testing. Not sure which is the worst of the two evils.