Actually, after I posted I went home and looked at all my settings, and indeed, I finally came across one setting which opted for maximum speed/spare the disk (which minimizes speed). When using the battery, it opted for maximum speed, but when it was plugged in, it opted for minimum speed. I reset it and it immediately began to behave properly again. I have no idea how that happened, I certainly did not do it knowingly. Someone may have hacked into my system.