How Datapump is faster than export/import backup?
Datapump is much more efficient than exp:

Datapump runs within the Oracle server processes, and can read directly from the database files and write directly to a file on the server. As I understand it, data access is direct, not via SQL.

exp uses the ordinary Oracle Call Interface (OCI) -- the library that Oracle provides to connect to the database. It can only run ordinary SQL statements like any other program.