Here is an uncomfortable result that deserves more airtime than it gets. In 2025 the research group METR ran a randomized trial with experienced open-source developers on real issues from repositories they already knew well. With AI tools allowed, they finished tasks 19 percent slower. Afterward, the same developers estimated that AI had made them about 20 percent faster. They were slower and felt faster, by nearly the same margin. That gap is the whole story, and it is worth sitting with.
Why fast can feel faster than it is
The researchers pointed at reduced cognitive effort. AI-assisted work felt easier, and we quietly file easier under faster even when the clock disagrees. Watching a model produce a wall of plausible code feels like progress in a way that staring at a blank editor does not, and the feeling is real even when the output is not saving you anything.
The catch is what the feeling hides. You did not write the code, so now you have to read it, and reading someone else's code closely enough to trust it is not free. If it is wrong in a subtle way, you pay twice: once to spot the problem and again to fix it, often after you have already convinced yourself it was fine. The generation was fast. The verification was not, and verification is the part that does not show up in the demo.
Before I am accused of doom: I should note METR themselves later flagged that their study design had problems, partly because the developers who benefit most from AI would not join a no-AI condition even at 50 dollars an hour. So do not read 19 percent as a law of nature. Read it as a real, measured case where the tool that everyone assumed was a speedup was not, and the users could not tell. That is the part that generalizes.
Where the time actually leaks
The productivity leaks are boring and specific, which is exactly why they are easy to miss.
- Reviewing confident nonsense. The failure mode that costs the most is a wrong answer delivered with total assurance: a made-up function that looks real, a plausible statute that does not exist, a config flag that was never a flag. Confident and wrong is more expensive than obviously broken, because obviously broken you catch in a second and confident-wrong you ship.
- Context-switching. Every trip out to the tool and back is a small tax on your attention, and enough small taxes add up to a workday where you were busy and moved nothing.
- The almost-right rabbit hole. The model gets you 80 percent of the way, and you spend longer chasing the last 20 percent through its logic than you would have spent writing the thing yourself. Nudging a nearly-correct output into a correct one can cost more than starting clean.
None of these feel like waste in the moment. They feel like work. That is the trap.
It still works, when you aim it right
I am not telling you to put the tools down. I use them daily and would be slower without them, which is precisely why I take the paradox seriously instead of waving it off. The tools genuinely help, but the win is conditional, and the conditions are learnable.
They shine when verification is cheap or the stakes are low. Boilerplate you can eyeball in seconds. A first draft you were going to heavily rewrite anyway. A language or API you half-remember, where the model jogs your memory faster than the docs. Throwaway scripts. Anything where being roughly right is good enough and checking is quick. In those spots the speedup is real and often large.
They quietly cost you when verification is expensive and correctness is non-negotiable. Subtle logic in unfamiliar code. Anything security-sensitive. Domains where you cannot easily tell right from wrong-but-plausible, which is exactly where the model's confidence is most dangerous, because you have no cheap way to check it.
The skill, and it is a skill, is noticing which situation you are in before you reach for the tool, not after. The developers in that study were not fools. They were experienced people who genuinely could not feel the slowdown while it was happening. That is the real lesson. The tool is not the problem and neither are you. The problem is that speed and the feeling of speed have come apart, and the only fix is to occasionally check the clock instead of trusting the vibe.