The Watermark delay metric is computed as the wall clock time of the processing node minus the largest watermark it has seen so far.
The watermark delay metric can rise due to:
1. Not enough processing resources in Stream Analytics to handle the volume of input events.
2. Not enough throughput within the input event brokers, so they are throttled.
3. Output sinks are not provisioned with enough capacity, so they are throttled.
Reference:
https://docs.microsoft.com/en-us/azure/stream-analytics/stream-analytics-time-handling