The problem is physics. We can't get to higher clock speeds with current materials, due to heat. It's kind of like how fighter jets haven't got any faster (top speed anyway) since the 60's...
Indeed, and first flew in 1964. Note that the follow-on MiG-31 was considerably slower despite sharing the general aerodynamic platform.
Since then Vmax has been declining, as the aerodynamics and mechanical complications ( e.g variable intake ramps ) of higher-Mach flight were determined to be less useful than transonic manouevrability and sustained supercruising.
The exception to this trend has been the superfighter category ( F-111, F-14, F-15, F-22, Su-27 ) which have maintained the same ~ M2.5 Vmax due to their specific role. Yes, even the F-111 was meant to be a fleet fighter.
But none have pushed up past the heady M3.0 level that was routinely broken by a series of prototypes in the 1960s.
With die sizes as small as they are, we have a problem where electrons...jump...through basically solid walls from an electrified wire to an unpowered wire. Now, turning on one circuit means the circuit browns-out and a neighboring circuit gets half-powered.
Fighter jets havn't got any faster because more speed is worthless compared to better avionics.
That is also what is happening with computers - it is simply more efficient to cram out more instructions per clock cycle than it is to cram out more clock cycles.
https://en.wikipedia.org/wiki/Mig-25