SOLR-7887 is done, switching to async logging will be a simple change to the config files for log4j2. This will reduce contention and increase throughput generally and logging in particular.
There's a discussion of the pros/cons here: https://logging.apache.org/log4j/2.0/manual/async.html
An alternative is to put a note in the Ref Guide about how to enable async logging.
I guess even if we enable async by default the ref guide still needs a note about how to disable it.....