2002-2007 xli? Yes it's underpowered. 2008-2014 xli? It has adequate power. 2014-2021 xli? It's body is bigger and heavier than 2008-2014 so with load it bogs down more than 10th generation but still all 3 models have adequate power to cruise on motorway comfortably between 120-140 and do northern routes as well
It's only when u have to do a difficult overtake (the vehicle you're overtaking decides to accelerate or there are slow vehicles hoarding space as such that you need a sudden burst of acceleration to complete the overtake safely)
If you're a speed freak or used to driving vehicles with bigger engines that don't need the same amount of throttle and revs to accelerate to cruising speed than yes xli's are underpowered.