That is the key. If you aren't using MQTT or other local OPC/UA servers, you need a scale-out architecture. Almost all normal OPC drivers for PLC protocols are extraordinarily latency-sensitive. Mainly because they don't buffer streams of requests and reply in streams, but handle requests one-by-one. When the network changes a request that normally takes 0.5 ms into a remote request that takes 20 ms, you crush your communications.