Character rigs used in cut-out animation are often very complex. Depending on their complexity and on the general performance of your computer, some rigs can take as much as much as 50 to 150 milliseconds for Harmony to render in the OpenGL preview of the Camera view.
When you manipulate an element in your scene, Harmony needs to regenerate the entire preview image for the Camera view. This means that if your scene has several rigs, the time required to render each of their preview adds up each time you manipulate something, causing delays of several hundreds of milliseconds per manipulation. As a result, it can be very challenging to work on scenes with several characters without using workarounds such as hiding all characters but the ones being worked on.
However, it is possible to manage this situation efficiently with very little compromise by using rig caching.
Caching is a mode that can be enabled on any node that outputs an image, but more importantly, it can be enabled on Composite nodes, allowing you to cache entire rigs in your scenes.
When a rig is cached, Harmony generates the OpenGL preview image for that rig once, then stores it in a database of preview images. Whenever the preview image for the Camera view needs to be refreshed, Harmony will simply re-use this cached image instead of regenerating the preview image for the cached rig.
Hence, in a scene with several rigs, you can enable caching on each rig separately, then temporarily disable caching on the rig you wish to work on. Harmony's performance will be comparable to that of a scene with a single rig in it. You can then re-enable caching on this rig, at which point Harmony will regenerate the rig's cache so that its preview image remains accurate while you start working on a different character rig.