As the efficiency of a logically irreversible computer approaches the Landauer limit, its speed must approach zero, for the same reason why as the efficiency of a heat engine approaches the Carnot limit its speed must approach zero.
I don't have an equation at hand, but I wouldn't be surprised if it turned out that biological neurons operate close to the physical limit for their speed.
EDIT:
I found this Physics Stack Exchange answer about the thermodynamic efficiency of human muscles.
Hmm... after more searching, I found this page, which says:
The faster the processor runs, the larger the energy required to maintain the bit in the predefined 1 or 0 state. You can spend a lot of time arguing about a sensible value but something like the following is not too unreasonable: The Landauer switching limit at finite (GHz) clock speed:
Energy to switch 1 bit > 100 k_B T ln(2)
So biological neurons still don't seem to be near the physical limit since they fire at only around 100 hz and according to my previous link dissipates millions to billions times more than k_B T ln(2).
At some point soon, I'm going to attempt to steelman the position of those who reject the AI risk thesis, to see if it can be made solid. Here, I'm just asking if people can link to the most convincing arguments they've found against AI risk.
EDIT: Thanks for all the contribution! Keep them coming...