On a properly functioning network, you are seeing significant delays in your on-demand reads and writes.
Scheduled reads (Group reads via Group threads or Device reads using your own threads) are overwhelming the controller by reading to agressively.
Rockwell Automation processors use overhead time, or background task time, to perform overhead tasks such as responding to a polled communications message request.
The overhead part of the scan is executed after inputs are read, ladder code is executed, and outputs are written.
First, ensure that you are requesting scheduled reads at the appropriate rate:
Next, it may be necessary to adjust the controller's System Overhead Timeslice
The A-B tech note G54059311 outlines how to increase the time or bandwidth in the processor available for communications.