Payroll was cut by more than half. Oil reserves jumped. The time it took to drill a deep-water well plummeted. Profits soared.
plummeted means a big decrease, usually associated with some kind of bad event. So the sentence means
the time it took to drill a deep-water well decreased a lot.
However, I find the paragraph to be a bit illogical. The paragraph basically says...
The salaries of the workers were reduced by more than 50%. As a result, the amount of oil that is stored instead of sold increased, and the time that it takes to drill a well decreased, resulting in increased profits.
Usually, reducing the salary of your workers by 50% will cause problems for your business, but everything mentioned in this paragraph is positive. The business now drills wells quicker and has increased their profits. Strange, eh?
Since everything is so positive in the rest of the paragraph, perhaps it would make more sense to say
The time it took to drill deep-water wells decreased substantially. This phrasing is more neutral than
plummeted, which is a word used in association with negative events, not positive events.