I think the abruptness of Louisa and Benwick's romance really helps with the pacing of the narrative, as well as conveniently extricating Wentworth from his entanglement with Louisa.

As it is the pace flags through chapters 13-17 during Anne's first weeks in Bath. All hope of a future for her and Frederick seems gone; first Benwick and then Mr Elliott fall by the wayside as possible alternative partners for her. Life is dull, there's not much to look forward to, the reader's interest droops with Anne's spirits.

Then the news of Louisa and Benwick's engagement explodes into the narrative like a bomb. Suddenly Anne is tinglingly alive with hope; our pulses quicken, we start to turn the pages faster. Events follow quickly - the walk with the Admiral - the meeting at Molland's - the night of the concert - building the anticipation, stoking up the tension, until the glorious climax of The Letter and the joy which follows.

If Louisa and Benwick's blossoming relationship had been treated at greater length, or news of it had filtered through to Bath little by little, the impact of the news would have been lost and the contrast between the dreary chapters and the build up to the final denouement would not have been so effective, IMO.