Replication WIndow is directly proportional to system load

As far as I understand, replication window is the time required to fully fill the oplog.rs collection. It is stated in the tutorial that this time is directly proportional to the system load. Should that not be inversely proportional given that as the load increases, the time required to fill up the log will be decreased?

Lecture: Local DB Part 2

Video Time: 03:14

Please confirm what the title of the lecture is and at what precise time it was stated?

Course: Cloud: MongoDB Cloud
Chapter_2_Replication
Local DB: part 2
Time: 3:25