Even if we say doubling instead of halving, I assume that the total portion of a transatlantic flight that it could travel at top speed would be pretty small, so the total time might still be more than half.

other than lower speeds during approach and departure around controlled airspace with speed restrictions, the overwhelming majority of a transatlantic flight will be at full speed.

of course, there are exceptions with congestion, hold patterns, excessive vectoring, etc, but this is generally true.

another thing to keep in mind is headwinds/jetstream. when going west across the atlantic; they can often be 100+mph. so the delta between boom and eg. a 787 becomes even more pronounced in this situation.

Typical height of the polar front jet stream is 30-40,000ft, that's great for a 787 that's most efficient exactly in that altitude range. But doing mach 1.7+ you want to be much higher for efficiency, so it's entirely likely that the Boom plane will miss the jet stream.

They're talking about going against the jetstream, so that doubling air velocity means more than doubling ground velocity. If the boom plane gets to miss the jet stream entirely that only improves the situation.

It’s not like the 787 can’t also make use of the jetstream.

A halving in speed would be surprising, at least.