/me mutters something along the lines of "Why didn't I think of that?" :)

Anyhow, I went as far as 2000x2000 and got the following results:

return hash takes: 32 secs
return hash REF takes: 11 secs
[download]

This demonstrates that hash REFs do indeed run much faster. I suppose that returning a regular hash requires that the interpreter copy the data structure... likely doubling the memory required to keep the data structure memorized.

Thanks a lot, I do believe I have got an answer to my "problem".