I have been using the free version of Duet Display to hook my iPad to my MacBookPro in order to have a second screen while on the road. This works great and there is virtually zero lag. Recently, I’ve started using Duet Display Pro to hook up an old iMac (late 2015) to my MacBook Pro. It’s a great way to use the iMac as the primary screen and the MacBook as a second screen. With programmes running on my super fast MacBook, I can now simply extend my screen to the iMac. The only downside of this construction is that the iMac does not truly operate in slave mode, e.g. notifications regularly pop up on my iMac and get in the way of the extended desktop, or my iMac decides to go to sleep while I’m in the middle of doing something. Not sure if Duet could resolve this, as it’s not a software issue. Other than that, lots of praise to Duet for creating this solution, and for their excellent customer service.