I have had this issue since the launch of BFA. I have a new alienware 17 laptop and could not figure out for the life of me why I was getting these spikes.
I opened tickets, I had friends that are I.T. professionals go through my router settings, computer settings, etc. Nothing stopped the lag spikes.
I went through and deleted everything that seemed useless on my computer and still nothing fixed the spikes.
However, I went into the task manager and started ending processes one by one. Finally after closing and ending my razer synapse application the lag spikes have completely stopped. I have not had one spike in about 2 weeks. I have no clue how a mouse controlling application is causing lag spikes but that seems to be the issue.
So if you are using a Razer mouse and are having big 5-10 seconds spikes I would try closing and ending the razer synapse application while you are playing wow.