This article may be a little biased but as a user of Gmail and Outlook I can agree that Gmail is sluggish to a certain extent. Outlook's interface made managing my email much simpler and straight to the point. I have not come across issues about Microsoft's server not playing nice with email clients. Yes Outlook may not have as many features as compared to Gmail but it does what it says on the tin, and does it well in my case.

I am going to have a look at Fastmail as an alternative to Gmail and Outlook.