I wondered why eM Client’s synchronizing had slowed down terribly for the past 2 months. Long story short. I had removed all Windows 10 apps except “Store”. I decided to reinstall Mail and Calendar. I went back into Default Programs and made sure eM Client was the default email app. Now eM Client synchronizes all my email folders at Outlook.com very quickly. Apparently it’s important to keep Mail and Calendar installed on Windows 10.
It was worth a shot and for me, it worked.