When you connect via the WAN are you still using PCoIP or is it RDP?
We are encountering an issue similar on the same configuration as you, the CPU spikes at 100% when you do anything from within the window such as Scroll, Drag and resize.
Have tried the mks.poll.headlessRate="1000 100 2" suggested in some threads and also the Teradici PCoIP updated server which according to the users has made some improvement but still not as smooth as I'd expect and arguably not what had been seen under 4.6
😞