If we think of encryption as playing table tennis where each person has just one bat and the pace is extremely fast, this is similar to asymmetric encryption as it uses one key.

Then, if we change the game and we give the players two bats, the first bat to stop the ball and then they must switch bats to return the ball, this would be much slower.

The same can be said for encryption; asymmetric is much more secure as it has two keys and uses Diffie Hellman, an asymmetric technique to set up a secure tunnel for the symmetric data. Symmetric encryption uses a block cipher where it encrypts large blocks of data much faster than the asymmetric technique.