Windows has a built in "project" functionality where you can use any other windows device as a secondary screen - not only it's super easy to use(press win+p, select the device, done) it's also very very smooth, and the other device appears as a native secondary screen in all settings panels.
The only limitation here is that it's WiFi only, and only on devices with an Intel CPU as it's using Miracast underneath. But I use a £29 Tesco tablet with a crappy Atom CPU for this and it's fantastic as a portable secondary screen that works without any cables and literally takes 2 seconds to set up.
The only limitation here is that it's WiFi only, and only on devices with an Intel CPU as it's using Miracast underneath. But I use a £29 Tesco tablet with a crappy Atom CPU for this and it's fantastic as a portable secondary screen that works without any cables and literally takes 2 seconds to set up.