2nd Line's First Response Time

Hi Ariel,
 
First response time in Zendesk will always be based on the first public reply by an agent. There's currently no native metric that could measure 'first response' after an assignee/group reassignment. But, if escalated tickets always receive the actual first response after it was assigned to the "2nd line" group, then you may try exploring building custom attributes and metrics. For example, you can try creating a custom attribute for timestamp of escalation (you might some ideas here or here on how to pull the Update - timestamp of an event), and then use DATE_DIFF to get the time from ticket creation until escalation. Form there, you can subtract the value of escalation time from the ticket First reply time; and, that should return the time it takes for the escalation group to reply from the time of assignment. 
 
But, if the '2nd line's' first response is not necessarily the actual first reply on the ticket, then I don't think there's an easy way to calculate this. You can try exploring with the earliest date functions, but with this kind of report inside the updates history dataset, custom calculations tend to be messy. 
 
Hope this helps. Thanks Ariel!