Originally Posted By: mlord
And it also affects some (all?) laptops as well, again on AC power, even with the old battery unplugged.

Reading instruction manuals or searching good sites would have revealed why. Apple has printed this in the instruction manuals for any portable Mac they made in recent history with user removable batteries:

Originally Posted By: Apple MacBook Manual
If the battery is removed from a MacBook or MacBook Pro, the computer will automatically reduce the processor speed. This prevents the computer from shutting down if it demands more power than the A/C adapter alone can provide.

Been this way dating back to the old Powerbook era at least.

The above statement was changed for portables that have a sealed in battery to inform folks how to monitor battery health. Modern macOS detects potential battery failures and presents the alert to folks when service is needed. Any time Apple has discovered a manufacturing fault, the entire batch of machines possibly impacted gets an automatic warranty extension usually out to 5 years for any battery issues. Or longer in the case of their iPod Nano battery recall.

I've come to understand it's part of Apple's commitment to triple bottom line practices, specifically focusing on the environmental cost of their products. Better batteries get swapped less, and it requires less wasteful tooling changes to manufacture common chargers to share among a line of laptops for several generations. Part of that compromise means that they plan on the charger hitting peak efficiency when charging the battery, instead of ensuring the charger has the output to supply the machine with efficient power during the extremely rare times a customer pushes the system full tilt. This requires pushing all the components hard to get a Mac laptop drawing supplemental power off the battery. As for how much, it's varied between every model. Several common Windows laptops do the same and have similar info out there about the practice, and I believe there's tighter integration with Windows these days to report the clock speed changes.

I think in my history of owning Apple portables, I've only had a few occasions where I've seen battery drain while plugged in. This includes times I had the old Powerbook running two instances of a game making use of the GPU, while other activities were keeping the single core CPU busy, and my professional experience of building software on various Macs.