Add Site Health test to break down server response time for uncacheable content

Feature Description

I'm running a membership site where most users are logged in most of the time. I also want to speed up the dashboard for authors. So page caching is not really an option for me. Instead I'm focusing on improving the server response time without page caching.

In order to improve the performance of my site, I would love to see a breakdown of the server response time, maybe something like the following:

I'm hosting on a cloud provider and I have options to scale up my webserver or MySQL database independently of each other. In fact most cloud providers have many options for hardware e.g.
https://aws.amazon.com/ec2/instance-types/
https://docs.microsoft.com/en-us/azure/virtual-machines/sizes
https://cloud.google.com/compute/docs/machine-types

I might invest in scaling up my hardware or switching to a different VM type/family if I knew what would make the most difference.

No implementation files, tests, or entry points are named. Begin by reviewing the proposed Site Health response-time breakdown and the existing issue discussion, then define the measurements, presentation, and completion criteria before implementation.