correct form is important weather your rep takes .3 seconds or 30 seconds.

rep speed can very greatly between different lifts. a heavy deadlift is gonna take longer than a power clean simply by the nature of the movement and the amount of weight used.
someone working speed sets is obviously purposely using faster rep speeds so their tempo will be different as well. a 1rm squat attempt will naturally be a whole lot slower rep speed than someone doing 12 reps of b/w chin-ups etc, etc, etc.

just use a natural rep speed that lets you maintain control of the weight and keep your form but dont slow it down simply for the sake of the "burn" or some other dipshit concept.

"Remember its HOW YOU LIFT not HOW MUCH OR HOW MANY"

that's just plain silly! how much, how many, and how often are just as important as HOW you lift. all need to be addressed for continued progress and avoiding injuries.

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