"Can't keep up! Did the system time change, or is the server overloaded? Running 2136ms behind, skipping 42 tick(s)" and "Something's taking too long! 'root.tick' took aprox 600.099637 ms"

I'm having a little bit of lagging in a single-player world. FPS goes from mid 40s to mid 20s when very little is happening.
I'm a little bit of a noob when it comes to the technical aspects of Minecraft, so bear with me on this.
Thanks.