If you watch the 1964 Kirov Shades w/Fedicheva & Soloviev, the tempo for the third “slow” (begins slow) soloist variation is the same as what was played during Doug Fullington’s Imperial restoration program at the Guggenheim several years ago.
Faster than played now at ABT and certainly at the now once-more Mariinsky.
Undoubtedly somewhat adjusted for individual dancers, but I guess only in the 1970s did the Kirov really start to slow things down.
You could say technically the variation is more difficult now–to start off very slow and then have to shift gears so dramatically.
But even as adagio was refined at the Kirov in the 1960s, ballerinas like Makarova and Osipenko (who’d excelled in that variation early in their careers) weren’t allowed to drag.