ARD has become very slow

I think I have a temporary work around. So the new iMac I have is has a 5k screen. The default aspect ratio is 2048 x 1152. Within display settings under System Preferences I changed it from "Default by display" to "Scaled" and choose the option that say's "Looks like 1680 x 945". This has drastically made a difference for me. It now works very smooth again! So my conclusion is it has to do with the amount of screen that is being processed through the connection is just too much.

I too am struggling with the SAME situation! I've also had to rely on using TeamViewer which is night and day in performance.

My situation seems to be the most noticeable when using a heavy application on the system in question. The app is ProPresenter 6 which is a presentation software. I have two external displays connected to a new iMac also. When PP6 isn't running ARD works but still a little laggy. Launch PP6 and a complete stand still. Launch TeamViewer and smooth as silk. The iMac is running macOS High Sierra 10.13.2. Connecting system is macOS Sierra 10.12.6.

Have you found or had any luck elsewhere with this issue?

Thank you - that did it!
It was driving me crazy too :-)