I can understand your thinking, but it could be as simple as giving the user the option to outsource the computation to a secure something or other, if their machine can’t handle it.

And yeah, the requirements are still quite high, but they are being reduced somewhat steadily, so I wouldn’t be surprised if average hardware could manage it in the long term.

Edit: For the record, Mozilla is one of the only companies I would trust if they said “the secure something or other is actually secure.” And they’d likely show actual proof and provide and explanation as to how.