300 ms is way longer than they budgeted; separately, I was alive then and it's a ridiculous claim, like, it takes a general bias we all have towards seeing the past with rose-colored glasses and takes it farcically far.
Don't want to clutter too much, I'm already eating downvotes, so I'll link:
On the average consumer hardware at launch, 95 and XP were slow, memory hungry bloats. In fact everything that people say about Windows 11 now was even more true of Windows back then.
By the end of the life of Windows 95 and XP, hardware had overtook and Windows felt snappier.
There was a reason I stuck with Windows 2000 for years after the release of XP and it wasn’t because I was too cheep to buy XP.
But I think the question was the other way: Why couldn't calc.exe launch in 300ms?