I tried everything. I broke up my 10k tags into different scan groups.
We changed the garbage collector,
We pinged from server to thin client at 1ms.
We wire sharked and found no packet issues.
We searched for inefficient scripts, and async/sync writes.
We called tech support.
Then we took the HMI out of the complication, we stopped the device on the gateway, made a new one, and only pulled 10 or so tags from it… still the lag of 6 seconds.
It was a port setting of 100mbps on one of our ethernet devices.
Good luck, maybe this google search helped.