One area where ActiveRecord can perform much better than Sequel is if you are selecting a lot of fields that you don't need, especially if those are date or timestamp fields.
Sequel typecasts all column values before yielding objects, while ActiveRecord doesn't typecast values unless necessary.
In both frameworks, you pay a penalty for selecting unused columns. However, that penalty is much greater in Sequel.
Of course, if this becomes a bottleneck, Sequel makes it easy to optimize by calling select with only the columns you need.