I have a 2016 MBP now and it runs League very clunky, but my old Iris Pro MBP ran it smooth as butter. Was curious if it's just how High Sierra runs games or if it's just the 2016 model, or is there a chance I just have a bad card in some way?
Some have reported poor gaming performances under High Sierra for recent laptops after waking from sleep. Try restarting and see if performance is improved.
A bad card is unlikely, and should manifest in graphics corruption. Any performance issue should lie in software.