I made a rather significant (for me) investment in a new PC when I started learning Blender. I wanted to make sure I could practice and work on any projects without having to either travel to campus to use the classroom machines or deal with hours-long render times on my existing PC. So I was really frustrated that my render times went from 4-5 seconds a frame to 20 minutes as I started adding elements to my scene(s). Thanks to this tutorial by CGMAtter, I was able to reign in those render times back to acceptable levels.