there seem to be some answers in regards to viewport vs render here https://blender.stackexchange.com/questions/2728/what-are-the-technical-differences-between-f12-render-and-viewport-preview and here https://blender.stackexchange.com/questions/143244/why-does-render-image-take-a-long-time-and-what-is-it-used-for i hope this helps...
awesome