how to set data in args from json api in onBeforeCellRender
The onBeforeCellRender implementation must be very fast because it's called for every cell in the grid during scrolling.
I recommend reading all cell data in advance and storing it on the client side. The onBeforeCellRender handler would only read the values from there.
The data can be stored in a global variable, or with resources (if it is resource-specific). See also the following tutorial: