We are currently using the Kepware IoT plugin to stream around 800k tags per second via MQTT ultimately into a Victoria metrics DB. It's a little clunky and requires more processing steps than I would like. I am currently moving away from this solution (since Kepware charges a reoccurring license fee every year for the IoT plugin), and toward using telegraf to read the OPC-UA data directly from kepware without the IoT plugin.

Do/Did you work at Kepware?