Mac OS menu clicking slow, numbers down the side I am running Mac OS 10.15.2. I rarely reboot my computer, but I had to turn it off to move it home due to Coronavirus, and I when I rebooted I found I had received an unwanted change.

Now, whenever I click a menu item (that is, the menus in the top left corner, for instance at the moment I am in Chrome so I have the items Chrome File Edit View History Bookmarks People Window Help), it gives little numbers on the left alongside each drop down menu. For instance when I click the File menu, I get a little 1 alongside New Tab, a little 2 alongside New Window, etc. 

And, clicking these menus is really slow. Even when I select something which does almost nothing (e.g. I click File, then change my mind and click File again to close the drop down menu), the little numbers stay around for about a second, and during that time I cannot do anything else. This ~1 second delay is really annoying, like I am running an overloaded or outdated computer (which I am not). Even worse, Cut + Paste takes several seconds.

I suspect these little numbers are the cause. So now for my question: how do I disable this feature?
You have accidentally enabled Catalina's new accessibility voice control feature, which lets you control the computer with your voice. The numbers you see are there to use as lables you can speak to exectute the commands. The same feature is also the cause of the slow-down in response. 
To disable the feature, open System Preferences from the Apple Menu, go to Accessibility and disable it from there.