We are the opposite here, no CPU problems at all, but still a massively slow EWS sync. I have not been able to get a hold of any customers I know use Apple Mail with EWS to see if it is the same with them. So right now, it appears to be related to eM Client.
I was figuring maybe it was some optimizations being done in the background causing the slow down, but it is has been almost a week and the issues still exist.
A fresh start of eM Client results in "Uploading items firstname.lastname@example.org/Inbox" and it sits FOREVER even though, there is hardly any emails in the inbox folder. One of the users said they never even noticed the syncing bar animation in eM Client until we did last weeks update.