Dmitri Shostakovich (right) photographed with Yevgeny Mravinsky who conducted the Fifth Symphony’s premiere in 1937
It’s not so much the tempo, and they vary considerably, not least Kurt Masur’s massive strides (LPO label), but more the rallentando that most conductors make on the final few bars (Mark Wigglesworth on his BIS recording an exception), a stratagem that is, it must be said, perfectly plausible. However, I believe there is no such marking in the score (but I don’t know), so when a performance maintains the chosen speed throughout the ultimate coda it seems to me to be exactly right. Try, for example, Vladimir Fedoseyev & the Hiroshima Symphony Orchestra (audio of Finale; from a performance on December 10, 2020), as direct as can be:
(Following which there’s an encore, a movement from Tchaikovsky’s ‘Mozartiana’ Suite.)
Here’s Mravinsky as a reference, from 1973, Leningrad; slower anyway and with some broadening at the very end.
Indeed, at the very end there is no broadening of tempo indicated in the score. Fedoseyev, as Colin says, judges it “exactly right”, an interesting clip. The broadening-out tradition, it seems, was right there from the beginning, witness the premiere recording under Mravinsky, 27 March 1938, presumably with Shostakovich on hand. Atrocious tuning. According to Alexandra Vavilina-Mravinskaya, the conductor’s widow, the orchestra was in such poor condition in the 1930s that the musicians were selected more for their political profile than musical abilities. https://www.youtube.com/watch?v=qz24-TXZxBQ. Many conductors make a meal of the ending, emphasising bombast at the expense of ensemble.
Vassily Sinaisky told me that Madame Vavilina-Mravinskaya was herself a big wig in the local Communist Party and that was why he steered clear of giving some late premieres particularly Babi Yar. It was too toxic for his reputation apparently. On the other hand Kondrashin had no such concerns and happily stood in. He, of course, later defected and a rumour persists he was assassinated in Amsterdam.