45mph doesn't actually sound very fast to me, especially going down.
It's not "fast", but in an elevator, the problem is not speed, it's air pressure. In an airplane that's pressurized, the cabin air pressure is set to achieve an approximate 500'/minute descent (the actual aircraft will be descending far faster than this) as this keeps passengers comfortable. Go faster and passenger's ears pop and they get mighty uncomfortable. Too fast and you can pop and eardrum.
That's the fundamental limit on the speed of an elevator - the passengers just can't take the air pressure changes without feeling rather queasy or having their ears pop annoyingly often.
Not sure how they solved this problem - pressurized building so the upper floors are closer to ground level, thus minimizing the air pressure differential?