There is some desire on the mailing list to create a separate logfile for slow queries. Currently it is not possible to do this cleanly, because the WARN level used by slow query logging within the SolrCore class is also used for other events that SolrCore can log. Those messages would be out of place in a slow query log. They should typically stay in main solr logfile.
I propose creating a custom logger for slow queries, similar to what has been set up for request logging. In the SolrCore class, which is org.apache.solr.core.SolrCore, there is a special logger at org.apache.solr.core.SolrCore.Request. This is not a real class, just a logger which makes it possible to handle those log messages differently than the rest of Solr's logging. I propose setting up another custom logger within SolrCore which could be org.apache.solr.core.SolrCore.SlowRequest.