How to slow down the mouse's double-click speed in macOS 10.14 Mojave 

To slow down your mouse’s double click speed

1. Open the ‘Apple Menu’ by clicking on the Apple icon in the Menu Bar.
2. Select ‘System Preferences…’
   
3. In the ‘System Preferences’ window, click on ‘Accessibility’.
   
4. On the Accessibility Preference screen, select ‘Mouse and Trackpad’ in the left column.
5. Adjust the slider for ‘Double-click speed’ to set a more comfortable speed.
   
6. When you are happy with your settings click the ‘Close’ (red) button at the top of the window or press ‘Cmd’ + ‘W’ to close the window.

Note: If this does not work it could be because your computer settings are managed by someone else (an IT department or administrator for example) in which case you will need to contact them to access these settings or for further help.