Hello BucRepus,
It is easy to be confused whether the numbers refer to bits or bytes.
Task Manager uses abbreviations like Mbps and Kbps - these are "bits per second" values.
Perform typically uses text like "Bytes Total/sec" - these are obviously "bytes per second" values.
1.5 megabytes per second is about 12 megabits per second - in the same ballpark as 20 megabits per second.
The values that you are seeing are almost certainly both "correct", but may be reporting on network traffic from subtly different perspectives.
Gary