MiG-25 and R-15B-300 engine: thrust, speed, limitations. Why was the MiG-25 so fast ?

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 The difference between the R-15B-300 and other jet engines, and its similarity to ramjet 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