How come serial performance is low

https://www.ibm.com/ibm/history/exhibits/mainframe/mainframe_PP1620.html

https://www.quora.com/How-long-does-a-computer-take-to-add-two-numbers

A raspberry pi adds 700m numbers in a second. A ibm 1620 does 2000 5 digit ads which is 20m. And the pi has pipe lining so the serial performance is 100x worse.

Why are modern computers progressed so little?