The speed difference is largely due to the iostream I/O functions maintaining synchronization with the C I/O functions. We can turn this off with a call to std::ios::sync_with_stdio(false);
By default standard C++ streams are synchronized to the standard C stream after each input/output operation.
Once synchronization is turned off, the C++ standard streams are allowed to buffer their I/O independently, you can try and see the time taken will be almost similar (possibly lesser than scanf)
solved why is cin/cout slower than scanf/ printf