Time from enqueue to start isn't a good metric - it completely disregards the queue size. Enqueuing 1M jobs won't change this metric as it only updates once the job reaches the front of the queue, and when the 1Mth job does that the situation is already over.

I had much better results with a metric that shows estimated queue time for jobs that are getting enqueued right now (queue_size * running_avg_job_processing_time / parallelism).