I have a hikari cp/mysqlconnector 5 patch from 2015 but internet says hikari is unstable i did not test it with 6 or later, here take a look
Edit to reply on your edit
Actually theoretically is (this is the reason I tried to test hikari in this patch), asside the code and how much you use it if the base is "crap" everything works with the "crap", I am not sure how many classes use database connections in a project, but lets say 40% of total classes most of them are on server load so lets take this out of the performance so the communications with the database after server load will be 5% of total project classes this 5% is crucial for server stability and response to clients so it might look small but it will boost every server/client interaction of every client, what do you think about that?
From my tests back in the days late 2014 I noticed big performance difference with HDD ofc (now on SSD its just milisecs), talking about player actions that cause the use of database, with timings sometimes reach 1-3 seconds for lagging but 0.3 and 0.2 msecs less was what I had achieved with live version of hellas and the test version via local network (stupid but wire and wireless tests), to sum up I think the connector can boost performances.