My slowdown occurred because without the battery the MacBook couldn't go full throttle, causing the kernel to churn. That's why the kernel usage in Activity Monitor shot up. Newer MacBooks are designed to operate on battery only, but not on electrical mains only.
What does any of this have to do with my iPad?
Zack said my battery had been cycled -- completely discharged and recharged -- only 16 times during the three years I've owned the MacBook Pro. It should have been cycled dozens of time, perhaps even 100, over such a period. Such cycling is critical to battery health; if you don't cycle the battery roughly every month, the dreaded memory effect occurs, causing the battery to calculate that it has less capacity than it does. The reasons have to do with the chemistry of some batteries, including the lithium-ion batteries commonly used today. In any event, the lack of cycling caused the battery's memory effect to kick in at an obscenely high level that essentially killed the battery.
That was the iPad's fault. I got my first iPad in 2011, not longer after I got the MacBook Pro. It didn't take very long before I stopped carrying the laptop with me, whether on business trips or to the living room. I'm iPad-only when on the go. That means my MacBook Pro is always plugged into mains power. The only time it had discharged is when I accidentally pulled off the magnetic power cord or the very, very few times I'd used the laptop as a laptop.
My iPad's battery is quite healthy, as is the battery in my 2011-era iPhone. The iPad and iPhone fully cycle at least weekly from my normal use. (No, their batteries don't last a week. I tend to top them off each evening, but there's usually at least one day each week where I drain the batteries to below 20 percent, which allows the battery to cycle.)
If I were using the MacBook Pro as a laptop, the battery would have been able to periodically cycle. But because I used the MacBook as a desktop, it didn't, causing the catastrophic battery failure that showed up as a superslow system. My next home Mac will be a Mac Mini, but that's easily a year or more away. Until then I've put a reminder in my calendar to cycle the MacBook Pro's new battery each month. (You can check the cycle count in the Power section of the Hardware pane in the System Information utility.)
I'm doing the same at work, since that mid-2012 MacBook Pro also never goes anywhere and thus never cycles. In fact, it's cycled only twice in the year I've had it! Fortunately, the memory effect on that battery has been minimal, so I'm in no immninent danger of a repeat catastrophe.
But cycling a battery is not easy on a MacBook. Apple's hardware is too good at not using power. For example, I left my office MacBook Pro unplugged for two days when I was out of the office, hoping to run down its battery to cycle it. When I got back, it still had a 75 percent charge -- so much for that cycling attempt.
You might think you could just unplug a MacBook periodically while working to run down the battery. But doing so causes the MacBook to stop sending its video signal to any external monitor (to save on battery life, of course), which means you need to open the lid and use the built-in LCD instead of an external monitor. I use large monitors at both home and work, and losing access to them while my MacBook is discharging is not a reasonable option.
[UPDATE: Since I wrote the article, I've found a trick that seems to work to let me cycle the MacBook Pro's battery while still using it with my monitor: Open the MacBook's lid before disconnecting the power cable. Then the screen stays on and continues to mirror to my external monitor as well.]
Apple should think about a way to build automatic cycling into its MacBooks, since it's so hard to manually cycle the battery. Because so many of us use iPads as we once used MacBooks, Apple has created an unexpected risk. After all, MacBooks make up the lion's share of Apple's Mac sales, whether or not they're used as laptops. It needs to find a fix that people can embrace.
The iPad is a wonderful device, which is why people like me use it exclusively away from a desk. But it shouldn't lead to our Macs being crippled.
This article, "How my iPad crippled my MacBook Pro," was originally published at InfoWorld.com. Read more of Galen Gruman's Mobile Edge blog and follow the latest developments in mobile technology at InfoWorld.com. Follow Galen's mobile musings on Twitter at MobileGalen. For the latest business technology news, follow InfoWorld.com on Twitter.