You’re getting progressively legacy (and more likely to be degraded) hardware. This impacts how tightly packed the instance type as a whole is, which impacts launch instance performance
Depends what you're trying to measure, I think? My goal as a FreeBSD developer is to look at FreeBSD performance -- this is both to show the improvements which have been made over the years and to alert me to any performance regressions (I generate these graphs automatically when I build the weekly snapshots).
If you want to compare EC2 to other clouds you would definitely want to use the latest instance generation, of course.
You’re getting progressively legacy (and more likely to be degraded) hardware. This impacts how tightly packed the instance type as a whole is, which impacts launch instance performance