Toogle for vfb missing

Hi…I’m facing an issue with my vray render window sometimes.
when you click on render, the render window which opens has these white boxes that move around and refine the image. although, for some renders those white boxes go missing and my render takes way too long.
im attaching an image so you can know what im talking about.
PS: if you could also tell me what those white boxes are called.

thank you:)