If you have ASM module enabled then you don't need an iRule as slowloris is already mitigated. LTM with an HTTP Profile configured on the virtual server will also effectively protect your backend servers (pool members) from Slowloris.
You can enhance the protection further by using a Slowloris iRule which will drop the clientside TCP connection if HTTP request is not received within a specific time slot after the connection is established (e.g. 1 second/1000 milliseconds) - this iRule can be found at the bottom of the KnowledgeBase article K10260:
