Does this make the server more responsive?

No, it is just an API for plugins to store things in memory.

When managed in memory rather than in storage in general
I'm sure it will be processed fast, but isn't there a benefit to storing the data in memory?

The only purpose of this plugin is to have a universal way to store and manage data across plugins instead of handling it directly in that plugin itself. Yes, storing in memory is generally faster than writing/reading to/from disk, but this plugin isn't going to magically make the server perform better as plugins generally already store their config/data to memory on server start.