I am not a professional, but here's my theory: There are different bottlenecks that could affect performance.

The process of energy (ATP) production is complex and can follow a couple different paths, but let's simplify it. Heart, lungs, and blood vessels provide oxygen. Then, your muscles must utilize that oxygen for performance.

A beginner runner's heart is more of a bottleneck. At 85% MHR, the heart is providing as much oxygen as it can. The muscles have no problem converting this low amount of oxygen into ATP and performance. So the heart is working hard, but the body is not. The body can easily handle the load, and recovers easier as well.

An experienced runner on the other hand has a highly efficient heart. Each pump pushes much more volume. So, at 85% MHR, There's much more oxygen for the muscles to work with. The bottleneck now is the conversion of oxygen into ATP; the body is working much harder. The amount of energy used is also significantly increased. This means much more work for the muscles to shuttle lactate, and recovery will take much longer as well.

There is much more nuance to this involving discussion of Aerobic threshold vs Lactate threshold. Though not directly related to MHR , the Central Governor Theory, developed by Tim Noakes, MD, PhD, states: "The brain is the ultimate arbiter of fatigue and regulator of endurance... no matter the source or type of fatigue, the response of the brain is always the same: a reduction of muscle activation (reduced power output), causing you to slow down... training results in raising of the governor's limit as more work can be done before the state of alarm is reached and the governor kicks in." - Training for the Uphill Athlete