A one-shot order delivers as fast as the provider and the service's daily speed cap allow — for a 1,000-like order on a service with no meaningful cap, that can mean most of it landing within minutes. Drip-feed spreads the same order across a longer window instead, delivering a portion at a time on a schedule you set.
The case for drip-feed is how it looks to anyone watching the account grow. A post that jumps from 40 likes to 2,000 likes in under a minute reads as exactly what it is; the same 2,000 likes arriving in batches of 100-200 over six or eight hours reads like organic engagement building up the way it normally does. For accounts that get scrutiny — brand pages, anything with an audience that might notice a sudden spike — that difference is the entire point of paying for drip-feed over the plain version of the same service.
The case against it is equally simple: drip-feed only matters when the delivery pattern itself is visible to someone. An order for a metric nobody's watching in real time gets no benefit from being slower, and a time-sensitive push — hitting a follower count before a specific moment, for instance — is actively worse served by delivery deliberately spread over hours.
Subscriptions are a related but different tool: instead of spreading one order over time, a subscription places a fresh order automatically on a fixed schedule — daily or weekly, for a set quantity, against the same link — for as long as the subscription runs. It doesn't detect new posts or react to anything happening on the target account; it simply repeats the same order at the interval you set, which suits a profile-level service (like followers) that benefits from a steady trickle over weeks, more than it suits reacting to individual new content.
Choosing between the three — one-shot, drip-feed, or subscription — comes down to whether the delivery pattern is something an outside viewer would notice, and whether the need is a single post or an ongoing stream of them.