If it's two different plans, that's probably the issue.

Make sure the same parameters are in use on both and make sure that both have updated statistics and defragmented indexes.

The very best way to compare apples to apples is to restore a backup from the first server on the second. That'll let you be sure that you have a precise copy of the data, the structures, statistics, indexes, etc. Then, any differences are likely to be caused by hardware; memory, cpu or disk.