queries response time fluctuation in MyISAM tables

Please clear me MyISAM normally utilizes CPU cores or not ? I thought it’s not because i notice queries execution time varies (differ) in different situations.
I have one slave handling read only queries from application and when no. of threads connected (low number of select queries) queries execution time is shorter as compare to when no. of thread connected higher (high number of select queries verified from show processlist) accessing different tables and all are serving i.e. no query in lock mode queries execution time suddenly increased to quite high.

From this scenario i am assuming MyISAM normally doesn’t utilizes full CPU cores while INNoDB normally do.

Please someone clear me about this because i notice queries execution time fluctuation dependent on no. of currently running queries.