Thanks for getting in touch!
This is indeed something which is possible in Email Meter Premium - you will have access to data up to 2 years before your original Email Meter sign up, meaning you can check your Monthly Reports and see how it changes month-to-month, both at the top of your report:
And in the specific Response Time metrics:
You can also generate a Custom Report for the entire time period, or year by year, to see long term changes.
Another super useful feature is Business Hours, which allow you to set up a range of hours between which response times are taken into consideration. These are applied retrospectively, so you can see how these affect previous reports, when you only consider responses within your work hours.
You can read all about our metrics here in our Help Centre, and if you have any more questions feel free to reach to me!