Eventually, over 10^100 years of operation you get into interesting problems with the number of possible states a mind of a given size has being finite; but if you can continually add more matter via non-energy-conserving interactions, you could add more memory/processing up to fundamental physical limits: you can only fit so much information in a given volume before it collapses into a black hole; you can only communicate at c, so if you make your mind bigger, it has to parallelize or slow down; Given the expansion of space, at some scale, light signals from one end of your mind can't reach the other (the space inbetween expands faster than C), which would be the the fundamental limit... except that gravity counteracts the expansion of space, and I'm not sure what the expansion properties of a visible-universe-filled volume of just-barely-black-hole-subcritical computronium is. I imagine you're in a big crunch regime at that point, except... Isn't that the same thing as a black hole collapse? Nonetheless, there's presumably some balancing density that perfectly avoids both expansion and contraction, though it's presumably unstable. Ignoring the instability-difficulties, though, that means you can continue growing the amount of memory you have by slowing down your processing speed to one clock cycle every ~2*the diameter of your mind in light-years years. I'm not sure what the ultimate limiting factor is here, given infinite time; does the amount of computation of this system in infinite time go to infinity as your scale goes to infinity, or does it asymptotically approach some finite upper bound?

It's the universe that kills us.