Optimally compressed data is essentially indistinguishable from random noise. Could we tell the difference between a universe that has already been converted into computronium and one that's empty? If you had an awful lot of matter and energy and wanted to make the most powerful computing device possible, would it end up looking a lot like a star? Nuclear fusion is a great energy source, and there's lots of thermodynamic information in the atoms and molecules that make up a hot plasma.