Searching a big mysql database with relevance
You should try sphinx search solution which is full-text search engine will give you very good performance along with lots of options to set relevancy.
Click here for more details.
Seems like the index doesn't cover Premium, yet that is the first ORDER BY argument.
Use EXPLAIN your query here to figure out the query plan and change your index to remove any table scans as explained in http://dev.mysql.com/doc/refman/5.0/en/using-explain.html
MySQL is good for storing data but not great when it comes down to fast text based search.
Apart from Sphinx which has been already suggested I recommend two fantastic search engines:
Solr with http://pecl.php.net/package/solr - very popular search engine. Used on massive services like NetFlix.
Elastic Search - relatively new software but with very active community and lots of respect
Both solution are based on the same library Apache Lucene