The evidence-led answer
NST verdict
For building muscle and general strength, how fast you move the weight barely matters across a normal range, roughly one to two seconds up and one to three seconds down. Grinding out very slow reps, ten seconds or more each, is worse, mainly because it forces a much lighter weight and fewer reps. Time under tension is not a number worth chasing. Control the lowering phase so you are not just dropping the weight, but you do not need an exaggerated slow negative. If strength or power is your goal, try to accelerate the lifting phase while staying in control and using a full range, though the trial evidence for that is thin.
Where "tempo prescriptions" come from
A rep has a lifting phase, a lowering phase and sometimes a pause. Tempo is how long each takes. The idea that a specific tempo, usually a slow one, builds more muscle comes from time under tension: the theory that keeping the muscle loaded for longer per set drives more growth. It sounds reasonable. It just has not held up as a training instruction when the thing measured is actual muscle or strength months later.
Rep speed and muscle size
A pooled analysis of eight studies found muscle size was similar across rep durations from about half a second to eight seconds, when sets were taken to failure. Deliberately very slow reps, ten seconds or more each, the super slow style, looked worse for growth, most likely because moving that slowly forces a much lighter load and far fewer reps, so the set does less work. There are few controlled studies here, so that is low certainty, but the direction is clear enough to act on. Zoom out, and an umbrella review pooling 44 systematic reviews of training variables concluded there is not enough evidence that lifting speed or time under tension meaningfully changes muscle mass or strength. Tempo is not one of the levers that has held up.
Should the lowering phase be slow?
Lower the weight under control; do not let gravity do it. Beyond that, a deliberately slow negative is not required. An older pooled analysis of 20 trials found that emphasising the lowering phase can add a little to strength and size, but put that down to the heavier loads possible while lowering rather than to slowness itself, and noted the gains were specific to the speed and contraction type trained. So control the lowering phase, do not drop it, but grinding a four second negative on every rep is not something the evidence asks of you.
Should you lift fast?
There is a theoretical case for trying to move the lifting phase fast, with intent, under control, if your goal is strength or power. In practice the trial evidence is inconclusive. A pooled analysis of 15 studies in older adults comparing fast intended with moderate lifting speed found no clear overall advantage for function, with a lot of variation between studies and signs of small study publication bias. Two distinctions matter. "Lift fast" means intending to accelerate the weight while keeping control and a full range, not throwing it, bouncing it, or cutting the bottom short. And for pure muscle size this is not a priority; it is a strength and power consideration.
Is "time under tension" a real target?
No, not as a number to chase. The umbrella review found no reliable effect of time under tension on size or strength, and the tempo pooled analysis found equal growth across a wide range of rep durations. Counting seconds per set is effort spent on a variable that does not move the outcome.
A sensible default
Rough guide, not a rule. Lifting phase: about one to two seconds, or faster with intent if strength or power is the goal. Lowering phase: about one to three seconds, controlled. No exaggeration in either direction, not a ten second super slow rep, not a dropped weight. Then stop thinking about it and put the attention on load, reps close to failure, and a full range of motion, the things that do move the needle.
What this means in practice
Lift at a controlled, natural cadence, roughly one to two seconds up, one to three seconds down. Do not slow reps down to chase time under tension, and do not let the weight drop on the way down. If strength or power is your goal, try to accelerate the lifting phase while keeping full control and full range. Spend the mental effort you would have spent counting tempo on load, effort and range instead.
Practical meaning
Lift at a controlled, natural cadence, about 1 to 2 seconds on the way up, 1 to 3 seconds on the way down. Do not deliberately slow reps to chase 'time under tension', and do not let the weight drop. If strength or power is the goal, try to accelerate the lifting phase while keeping full control and full range of motion. Put the attention you would spend counting tempo on load, effort (reps close to failure) and range instead.
Limitations and uncertainty
The 'super slow is worse' conclusion rests on few studies. The hypertrophy pooled analysis used sets to failure. The best pooled analysis on lifting fast is in older adults and measures functional capacity, not size or one rep max in trained younger lifters. The lowering phase pooled analysis is from 2009. Several syntheses share author groups. These findings are about muscle size, strength and power, not sport skill or health.