How much computational power fits into a cubic centimeter?
The statement I made wasn't particularly well written. I was referring to a combination of the Margolus-Levitin theorem (which gives the minimum time to move between two orthogonal states, and hence a lower bound on the time required to perform a gate) and the Schwarzschild radius (which gives the minimum radius of a system of some fixed energy before it forms a black hole). Combining the two of these leads to an upper bound on the number of gates which can be performed in a fixed region of spacetime. You can think of this as the maximum number of gates per unit time which can be performed.

The idea to combine the two theorems comes from Seth Lloyd, who uses such an approach to show limits on the computational power of the universe(link). He also has another paper where he looks at the maximum performance that can be achieved by a 1kg computer of volume 1liter  (something he calls the ultimate laptop and which is almost exactly your question). See arXiv. He obtains a limit of $10^{51}$ operations per second on $10^{31}$ bits.
Due to the finite age of the universe and the finite speed of light, when combined with Lloyd's results, you obtain a constant upper bound on preprocessing.