Control iMac using Macbook Air keyboard and mouse? I want to use my laptop keyboard and mouse as the input for both the Air and the iMac. I have tried apps that let me use the same input devices across different computers via wifi, but found them laggy.

Can I use ethernet to do this instead?

Or do I need to basically just use my iMac as a dumb monitor (with a thunderbolt cable I assume?). My iMac is really a great fast computer and didn't want to lose it and make the Air do all the heavy lifting if I can avoid it, I just prefer to have the second screen in front of me (ie laptop below and in front of iMac) and use the comfortable keyboard/trackpad setup of a laptop...
If you are fine with using your iMac as a display with your MBP doing all the work, enable Target Display Mode.


  
  Make sure that your iMac is turned on, and the other Mac is logged in to a macOS user account.
  Connect the Mini DisplayPort or Thunderbolt cable between the two computers.
  Press Command-F2 on the keyboard of the iMac that you want to use as a display.
  


If you want to your iMac to do all the work, you could use thunderbolt networking and then use some of those apps that you tried earlier with wifi. To set up thunderbolt networking, just follow the directions here.