Performance Bug in Trigger Causing ~1000x Slowdown
Nice find. Thanks for posting. Yes the inner join is generally better than a subquery.
To answer your secondary question, to get it merged you need to create a ticket at https://lab.civicrm.org/dev/core/-/issues/new and then if you're able to also make an associated pull request at https://github.com/civicrm/civicrm-core against master. See also https://docs.civicrm.org/dev/en/latest/tools/git/#contributing. Then someone needs to confirm and test it.
When you do post your ticket, it would help to also describe the parameters (e.g. # of activities in the database, any other relevant variables...) so that others can reproduce/evaluate.