That behavior does sound like what happens when you start using too much memory (non-linear reduction in performance). So you should check the memory consumption of the process during different points in the run (for example, via the "ps" command).

Some file systems can also get much slower when they get nearly full so that is another possibility to check.

Another possiblity is that your total (virtual) memory consumption hasn't grown too high but that you are jumping around too much in memory such that your "working set" is too large to fit in the available RAM.

But, yes, we are all just guessing.

Nothing in your description rings alarms bells to me about where the memory usage would be growing.

        - tye (but my friends call me "Tye")