A lower maximum possible speed, with a fixed maximum thrust. But, not necessarily a lower optimum cruise speed or economy. If weight is higher, lift is higher; then, drag is higher, and if thrust cannot go higher, you would be right. You may not be cruising at maximum cruise thrust with a lower-than-maximum weight though. And, if the weight is centered properly with respect to CG limits, the AOA may not be changed enough to warrant a lower economy at a higher cruise speed compared to a lower weight.