My assumption is that ASIC miners optimize for the "hashes per joule" metric, as did GPU miners over CPU miners.

It would seem that after some time, the decision to mine Bitcoins would be based on if you could source energy cheaply enough.

When do we reach that point?

I'm not a hardware guy so I'm curious to understand what the upperbound limits are of ASIC mining from a technical perspective. How much more can we optimize?

Thanks!