Why was the MiG-25 so fast? thrust, speed, limitations.

The video about the Tumansky R-15B-300 engine on my channel describes its main characteristics, but left questions: why does it give huge thrust at high speed, what do the words mean that the R-15 works like a ramjet at high speed, and so on. This video talks about these and other things.

0:15 A distinctive feature of the engine R-15B-300
3:27 Comparison of engine thrust
4:58 Top speed at an altitude of 18 and 20 km
5:39 Flight duration at full afterburner
7:23 About the aerodynamic design and weight of the airframe of the MiG-25