Variable bit rates with "-vb" and "minrate"/"maxrate" settings in FFmpeg

For rendering, the time it takes is very different.

• -b 3500K -minrate 0K -maxrate 8000K takes 1hr 17min


• vb=3000k takes 2.5 hours

What is vb 3000k and how is it different from -b 3500K -minrate 0K -maxrate 8000K? Are these variable bit rate settings?

If I changed to -b 3500K -minrate 3500K -maxrate 3500K, is that constant bit rate?