1. The Issue

Having the LAN adapter connected while in Sleep Mode causes network slowdowns due to a bug with the USB LAN adapter's OS built-in network driver.
 

OS : macOS Catalina (10.15), Big Sur (11.0-11.2)
Monitor : FlexScan EV3895, EV2795, EV2495

2. Solution

Please update the macOS to Big Sur (11.3) or newer to solve this issue.