I'm trying to write an excel formula to calculate the performance improvement
using timestamps.

Process A took 2hrs, 20 min, 9 sec expressed as 2:20:09
Process B took 1hr, 12min, 28 sec expressed as 1:12:28
Difference is calculated as a-b = 1:07:41

How do I calculate the Process B percentage improvement in processing time
using these timestamps?