Do breast implants sag quicker if they're placed over the muscle?

Implant sagging is less a function of implants but rather soft tissue/skin quality. That being said, I do believe a submuscular implant does benefit from greater relative support in the lower pole.

Aesthetically, implants placed superficial to the pectoralis major create a rounded, convex appearing breast profile. This effect is camouflaged, at least initially in larger breasted patients. However, as a woman ages fat atrophies and breast tissue descends. The result is a more noticeable implant specifically in the upper pole. Similarly, patients who have thin coverage superiorly are more likely to be able to perceive the implants and at higher risk of visible rippling.

As always, discuss your concerns with a board certified plastic surgeon (ABPS).

Breast implants probably sag similarly whether over or under the muscle. The muscle does not really hold them up. Sagging is often caused by skin quality and other factors. If a patient is going to have sagging breasts, often the implants stay in a good position and the breast tissue sags off of them.

In my professional opinion, placing implants in a subglandular/subfascial position (above the muscle) does not provide anywhere near as adequate support compared to a properly dissected submuscular dual plane pocket.

Dear mfourie51,
I almost always place implants submuscular. It lowers the rate of capsular contracture significantly. In addition, it looks much more natural because the muscle provides covering over the implant so its not as round on the top. I've also noticed the implants drop less over time when they are protected under the muscle.
Daniel Barrett, MDCertified, American Board of Plastic SurgeryMember, American Society of Plastic SurgeryMember, American Society of Aesthetic Plastic Surgery

Yes. When implants are placed under the muscle, the muscle provides some support and thus reduces the tension on the breasts.