When popup data update takes too long, there is a preloader in popup to indicate, that data is not yet updated.
The problem is that in order to display the preloader all popup content is removed and then preloader is displayed. This causes popup to shrink to preloader size, and then expand again to contain new popup content. This creates a lot of flickering.
Suggestion is to gray out current popup content and show preloader over it, instead of removing the content.
This way popup will be resized only to fit the new content, thus decreasing the flickering.

Preloader should also be added on popup submit button, when it is pressed.
Preloader (with delay) should also be added on graph preview in Graph widget configuration popup. (graph is updated every time field in configuration is updated).