Jack Dorsey just discussed this concept a bit in an interview on Bill Simmons' podcast. They were discussing the pitfalls of giving users (or others) the ability to edit/delete their tweets.

In discussing having a N-second window where a user can correct typos, Dorsey brought it down to a tradeoff between real-timeness and correctibility. Essentially he feels that much of Twitter's charm as a platform comes from the fact that updates feel "live", so every delay from tweet source to delivery needs to be given a heavy cost in the cost/benefit analysis.

Adding a delay like that as a feature (especially for a figure notable enough to move the stock market with a tweet) would undermine that message completely, by preventing that figure from engaging with the conversation in real time.