The Mikoyan-Gurevich MiG-25 is a supersonic interceptor and reconnaissance aircraft that was among the fastest military aircraft to enter service. It was designed by the Soviet Union's Mikoyan-Gurevich bureau.Wiki
But the MiG-31 coupled with the Zaslon RaDAR and the long range MACH 4.5 R-33 missile has the ability to engage the SR-71 from long range in frontal-aspect and short to medium range in rear-aspect. This capability is further enhanced with today's MiG-31M/BM which carries 6 extended range MACH 6 R-37s and an improved Zaslon-M RaDAR. While the MiG-25s incapable at best at defeating the SR-71 with its low-performance missiles, the MiG-31 is a real danger to the SR-71 and is a reason why the U.S. stopped SR-71 flights over Russia: theaviationist.com/2013/12/11/…
Today's MiG-31M/BMs also have LERXs to improve maneuverability and high AoA handling. Not F-15 kind of maneuverability but still much more than sufficient to perform notching and cranking maneuvers to defeat missiles fired from long range.
With that said, the US lost it's only easy survaillance over Russia.
The Foxhound has nothing to chase anymore, it's original targets now at museums. Well, it's just an versatile jet waiting for prey... until Mother Russia sends them off to hunt those who threaten her.
As par with the F-15, the F-15 has an advantage over the Foxhound in terms of maneuverability, but it will have quite the hard time due to the Foxhound's Zaslon and it's thick armor. Plus, the Foxhound is able to do Beyond-Visual-Range attacks (BVR).