I've been playing Minecraft for quite some time now, and I remember in previous versions of Minecraft (I don't know if it still exists) that there used to be a graph on the screen when the player pressed F3, called the "Lagometer" I never understood what the graph detailed, and would like to know specifically.
The lag graph still exists (with a slight change to how it looks); you can see it by pressing ALT + F3.
The graph shows how long each frame took to render. The higher the bar, the longer that a frame took to render.
The numbers along the side should be taken as 1/number of a second that the frame took to render. If your data is around the 30 line, then frames are taking 1/30 seconds to render, and you will be able to get around 30fps.