Mysql query with Left Join is too very slow

Query:

   select `r`.`id` as `id` 
     from `tbl_rls` as `r` 
left join `tblc_comment_manager` as `cm` on `cm`.`rlsc_id` != `r`.`id`

Both tables have 8k records but why is it very slow, taking 2-3 minutes and more sometimes?

OMG , this query makes mysql server down. Will get back to you peoples in a second :(

All peoples those suggested Indexing the columns are all Correct. Yeh the query i wrote was silly and buggy. Thanks correcting me.