Start with the process Name using the highest % of one of the listed (CPU, Memory, Disk, Network) resources.
You can right click that process Name for more information.
May be tricky as the values may change and jump around as various processes share the resources.
You are looking for a process that, when it is using an increased % of some resource, you also see a slowdown in network performance.
Resource Monitor does much the same as Task Manager but provides a graphical representation and additional data that will help narrow down the culprit.
Your objective is to identify some process that either consumes network bandwidwith or seems to impede network performance.
Then the next step is to learn what that process is, what it is supposed to be doing, and what it is doing: E.g., some buggy app trying to "phone home" and gives up after x amount of time or attempts.