It doesn’t make much sense for the SLA timer to run on tickets where the agents are waiting for the customer’s reply or for information from a third party to proceed. In such cases, the ticket status can be updated as 'Pending' (or any other custom status for which the SLA timer is off depending on the use-case), so agents aren’t penalized for something they don’t have any control over.
All these tickets which are in statuses where the SLA timers have been turned off are called On Hold tickets. This way, you can eliminate the on hold tickets when you are looking at performance reports.