I read Permutation City by Greg Egan

It‘s 2050 and humanity has the technology to scan brains and run them in a virtual reality—at 17x slowdown. Paul Durham, or rather a copy of him, finds himself in one of these environments, proving a theory of his. Will it work? And what is the meaning of the Autoverse, a crude digital replication of atoms and elements virtualized on a computer?

This is one of the books that leave you with lingering thoughts and concepts—of reality and time itself. I loved how the concepts and their limitations are explained, instead of being stated as facts. If you like science fiction, highly recommend!