Howdy!
The DRBD solution for Chef HA is in fact EOL. The current architecture for Chef Backend HA is more modern.
https://docs.chef.io/install_server_ha.html
This sort of set up can handle many thousands of nodes accessing the server on regular intervals. For smaller installations, single instances with robust backup policies are often fine. The tiered architecture allows for additional controls if your hosts are in multiple physical or network locations and you'd prefer to lock down access to the data components. Overall performance is also related to the Chef features you choose to use, such as whether or not you are utilizing policyfiles (which remove dependency solving from the Chef server).
If you have a large environment you'd like to run Chef in, our customer success engineering folks can help with sizing and planning.