The issue is that while many requests are very fast to serve from cache, they can get blocked if your limited number of workers get caught handling long ones.

The issue is that while many requests are very fast to serve from cache, they can get blocked if your limited number of workers get caught handling long ones. If an admin is using a rarely used and inefficient page, or a few too many people are uploading lots of images at once. I have 400 real estate agents, and if they happen to all want to edit their listings at the same time then the customers get frozen out. Varnish would help here.