You're assuming performance has been the core priority, or even a priority at all, and I think this is a bad assumption to make. I would estimate a much smaller number of people-months of work if I were you.

Dev users assume the only problem a product can solve is performance, when there is a lot more than that in reality.

Maybe in the past companies wouldn’t take the extra time for performance enhancements - but they’re apparently saying that AI is sooo good and speeds up work that they don’t need all of these extra people. So if their product was sped up it would enable their customers to work faster and lay off all of their extra employees (or just keep everyone and just do more stuff faster).

So are they doing this to make the product better or, as others have mentioned, they can’t innovate further and can’t grow their market so they need to cut costs.