The ending of the story is 'quick' as the reader is left to contemplate their own interpretations of the clues that Holmes was faced with, and their moral reaction to the death of Dr Roylott as the result of his own murderous machinations.
Conan Doyle's intention, like Poe before him, was to present the reader with evidence to evaluate through the story, followed by an ingenious explanation linking the real clues together to a creative and engaging conclusion. The purpose of the story is to ascertain the meaning of 'the speckled band' as the dying words of the first Miss Stoner. Once the reader has worked out their solution and compared it with Holmes' conclusions, Conan Doyle's instruction as a writer is done: all that remains are the moral implications whose explanation is left to the reader.
According to the great American genius Edgar Allan Poe, what is important in a short story is to create a "single effect." Poe uses the term "effect" a lot. What he means is a feeling or an emotional effect. It seems obvious that once this effect is achieved, the author should terminate the story as quickly as possible so as not to dilute the emotional effect with extraneous data. The reader should be left with a "feeling." That is the purpose of a short story.
In the case of "The Adventure of the Speckled Band," Sir Arthur Conan Doyle very artistically and adroitly provides much of the follow-up information at the beginning rather than after the final effect has been achieved with the sight of Dr. Roylott having died an agonizing death and still having the speckled band wrapped around his forehead. Doyle moves the ending to the beginning as follows:
Of all these varied cases, however, I cannot recall any which presented more singular features than that which was associated with the well-known Surrey family of the Roylotts of Stoke Moran.The events in question occurred in the early days of my association with Holmes, when we were sharing rooms as bachelors in Baker Street. It is possible that I might have placed them upon record before, but a promise of secrecy was made at the time, from which I have only been freed during the last month by the untimely death of the lady to whom the pledge was given. It is perhaps as well that the facts should now come to light, for I have reasons to know that there are widespread rumours as to the death of Dr. Grimesby Roylott which tend to make the matter even more terrible than the truth.
The real facts of the case did not come to light at the time they occurred. Roylott's death was explained as having been caused by careless handling of a pet snake. There was no need to taint the family reputation by accusing him of attempted murder of his stepdaughter Helen. It would be impossible to prove, anyway. As far as the murder of Julia was concerned, that was a closed case and Roylott had gotten away with it. But he was appropriately punished two years later by being killed by the same snake he had used to kill Julia. There would have been no point in reopening that case in order to convict a dead man, and it might have been impossible to connect him to Julia's death after all that time.
Helen will get all the money rather than only one-third and will soon be happily married.