Scanclass max speed

It depends on load. Look at the actual statistics for the specific device, particularly the average response time. That's the round trip request from Ignition/response from PLC. Then look at the load factor. Anything close to or above 100% means there's not enough bandwidth (somewhere, usually the PLC) to satisfy all of the subscriptions at the requested pace (scan classes), slowing everything down. The more tags monitored, the slower it all gets.
You can use the system.opc.* functions to inject additional requests on demand -- those will perturb the statistics when used, but will execute as soon as possible. Round trip times from a gateway script using these functions will be notably faster than from a client script, of course.

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