Hi everyone and Happy New Year. I've been using a 2015 rMB for a year. Overall I've quite happy with it, except for MS Excel which has always been kinda slow compared to my previous Windows experience. Just a slight lag when clicking, and a few bugs here and there which made me seriously consider switching back to windows. But the real issue is that since a couple of weeks, Excel has become so slow it's barely usable. The rainbow spinning wheel is always showing up for 5 seconds each time I interact with anything. Then I noticed that it's not happening when there is no screen hooked to the Mac. I'm using the Apple HDMI multiport, so I tried with a cheap HDMI dongle I also have, and the same happens. I'm hooked to a 2540x1440 screen, which otherwise works perfectly with all other apps. I'm using Excel 15.29.1 on Sierra latest update 10.12.2. I'm not sure if this issue followed a particular update of MacOS or Excel. And the RAM or CPU remain at a normal load while the issue occurs. I'm going to try reinstalling Office 365 just to check, but I'm curious to know if anyone has experienced the same behavior lately, and if anyone has an idea to fix this. Thanks --- Post Merged, Jan 2, 2017 --- Ok I just reinstalled Office 2016, without even uninstalling the previous instance and it seems back to normal. Still kinda slow, but usable. Still curious to understand what happened though. And quite disappointed with how Excel overall performance on mac (this is my first). I would consider upgrading to a MBP but I'm not even sure it would change anything since it is obviously not a lack of computing power, more a bad coding of the app. MS should do better unless they want us to switch to windows, which is always an option. I will also consider switching all the way to google apps though, because I like this light little machine.