If true, you should have seem a BIOS alert at some point when first turning on the computer, telling you your charger was working at a reduced wattage.
If not that, then you had a bad wire in the AC or DC side of the power pack. That’s pretty dangerous. Look for an exposed wire or crushed/folded point. Also, a slight misalignment of the cable that plugs into the AC adapter could make less than optimal contact.
No alerts of any kind. I just figured it is a old laptop, and that was its normal speed. Now running faster than I can think.