Workflow Task Scheduled but took so long to start


I have some workflows with status WorkflowTaskScheduled, but it took minutes even hours until it is WorkflowTaskStarted. As shown in the images, a workflowTask was started at 9:19, but it was just started at 10:26, around 1 hour 7 minutes since initialization. Another workflow was scheduled at 10:25, and was started at 12:25, 2 hours since initialization. This causes domino effect as the other transactions are also scheduled, waiting to be started.

I am wondering what are the possible causes of bottlenecks? Where can I find the number of workers and pollers working on the tasks? Is it possible to see how many tasks are there in the queue?

Thank you in advance.