What Is the Primary Measure of “Network Congestion” in a Blockchain?

The primary measure of network congestion is the backlog of unconfirmed transactions in the mempool and the resulting increase in average transaction fees. When the number of pending transactions exceeds the capacity of the blocks being mined, the fee required to get a transaction confirmed quickly rises.

High average fees and a large mempool size are the most direct indicators that the network is congested.