It was the cable all along!
My computer was running slow when I was editing videos with the Adobe Premiere Pro and the other Creative Cloud apps. I decided to install a new Solid State Drive that I ordered from iFixit. The new drive would work as an external drive plugged in through the USB, but it would work horribly when I installed it internally. The screen would flash between the apple logo, a crossed out circle, maybe even a question mark, if my memory serves correct. If it would finally boot, it would be suuuuuper slow. I tried a number of solutions and figured it might be the hard drive, but, alas, as the title suggests, it was the cable all along.
The replacement took like 15 minutes. I had dug into my computer's insides a couple times before when I had been replacing the hard drive. This was just a couple extra steps. Super easy. One tricky part, however, was sticking the cable in the right place. Part of it works as a sticker - you peel off the back and slap it down. You just gotta be careful to have good placement. Basically, just have the same placement as the cable you remove.
Follow the instructions. They're great!