I have a question about server performance.

has different standards. Can be be the availability and speed of response was a benchmark may be. There is no set standards of all.

I don't know exactly which server you're talking about. Are you talking about the web application server, are you referring to various other daemons, or are you talking about the equipment itself?It's ambiguous.

Connection pools have a large overhead of connecting to the data surface, so they are connected in advance and used by borrowing those connected connections. In other words, it's not closed after use, but the concept of returning it. I don't know what normal DB API means, but it's quite fast to use the connection pool rather than creating a connection on demand.

It has to depend on the amount requested by the application. There can be less than 20 small services. If you take a large connection pool, that's how much more resources you use. Of course, you have to profile it to find the best value.