What would be the next step after quantum computing?
This is mainly a philosophical question and depends on your model of reality. Under some assumptions, time trevling ability will allow aditional speedup.... You can read about the CTC model, mainly introduced by Scott Aaronson, that deals with this subject.
https://www.google.com/url?sa=t&amp;source=web&amp;rct=j&amp;url=https://www.scottaaronson.com/papers/ctchalt.pdf&amp;ved=2ahUKEwih8a_Qz-LnAhUCGewKHUFlBZMQFjABegQIBhAL&amp;usg=AOvVaw3TMaefh-UTmCsAHKRT6EhL