When total training volume is held constant, varying the duration of the eccentric phase in resistance training does not produce a statistically significant difference in muscle hypertrophy.

Study: Effects of resistance training on hypertrophy, strength and tensiomyography parameters of elbow flexors: role of eccentric phase duration

When people lift weights slowly on the way down versus normally, both groups ended up with the same amount of muscle growth — so slowing down the lowering part doesn't make your muscles bigger, as long as you do the same total amount of work.

Study: Effects of resistance training with controlled versus self-selected repetition duration on muscle mass and strength in untrained men

When people lift weights and lift the same total amount of weight, it doesn’t matter if they lower the weight slowly or quickly—their muscles grow about the same.

Study: The effect of eccentric phase duration on maximal strength, muscle hypertrophy and countermovement jump height: A systematic review and meta-analysis

Slowing down the lowering part of a weightlift doesn’t make your muscles grow bigger than doing it at a normal pace, as long as you’re lifting the same total weight. The science says both ways work about the same for building muscle.

Study: The effects of eccentric phase tempo in squats on hypertrophy, strength, and contractile properties of the quadriceps femoris muscle

Slowing down the lowering part of a squat led to bigger leg muscles than doing it fast—even when people lifted the same total weight. So, tempo does matter for muscle growth.