How to identify bottleneck of data refresh rate?

I use OPC foundation .Net stack to subscribe 500 nodes from a Ignition gateway within VPN.

the timestamp of data received is 2 minutes behind current PC clock time.

Don’t know what is causing this 2 minutes delay, PLC? SCADA? OPC Gateway? network congestion?
How do I identify the problem?

Have you checked to make sure the clocks on the two machines aren’t just two minutes apart from each other?