Hi Ozy smile. AWS IoT Core supports up to 100 messages per second per MQTT client connection. Please see limits here: https://docs.aws.amazon.com/general/latest/gr/iot-core.html#message-broker-limits . In particular "Publish requests per second per connection".
So one message every 3 seconds is not, in general, a problem. However, much depends on what exactly you mean by "real time device monitoring". There will be some time taken for messages to traverse the (vagaries of the) internet and reach IoT Core, plus some additional time for messages to propagate through IoT Core, rules engine and so forth before you act upon them.
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