Why is my Nintendo Switch Charging So Slowly?

Different cables and USB wall adapters will provide different power output.

Try using the original dock and your charging experience will increase significantly.

As you say you've had the Switch three years, it may be time to clean out the USB-C slot with a toothpick.

USB-C sockets seem unfortunately prone to collecting dust, lint, and other kinds of gunk. You can sometimes tell this is becoming a problem if your charging cable "mushes" loosely into the charging socket instead of clicking in firmly.

A toothpick is a reasonable way to clean it out. Don't push hard enough to break the toothpick off in the slot. You'll still probably break a toothpick or two (it'll just bend at the end); when this happens, use the other end or discard it and get another toothpick.

I had a similar problem with my phone recently, which went from charging completely in 2 hours to only charging about 3-5% per hour, and eventually not charging at all. After scraping out quite a lot of pocket lint with a toothpick, the USB-C cable now clicks in firmly again, and it's charging good as new.

USB-C throws most people on power requirements because prior versions of USB "just worked" when you plugged them in (i.e. micro USB, which is what most Android phones used until the last few years)

The base USB-C uses the older USB standard of 1-2 amps @ 5V. You'll note even the standard Switch chargers offer that. The reason for that is that immediately offering more amps or voltage could destroy the device. You can plug any USB-C device to a Switch charger and get a "slow charge" of 1.5A @ 5v. The inverse is generally true as well (with some caveats below).

If you do own a USB phone, however, odds are good they gave you one that will charge your phone faster than that. Qualcomm makes one called Quick Charge for phones, but the Switch uses something called USB-PD, which isn't the same (i.e. plugging your Switch to a Quick Charge device won't charge at a higher rate).

The catch is you need two things to do this

1. A charger that supports the USB-PD standard
2. A USB-C cable that fully supports the charging standard

Your device and the charger then negotiate a higher power delivery. The problem is there's lots of cheap non-standard stuff on the market that can fry your devices (especially a Switch). Exhibit A would be an early Nyko Switch charger

While a wide variety of third-party chargers and cables have been linked to Switch bricking, the most common complaints seem to center on a third-party dock made by Nyko. Like many cheap USB-C chargers, the Nyko dock does not actually have a dedicated PD controller; it instead implements the PD protocol using a general purpose microcontroller—and that microcontroller was sending 9V to the Switch over the CC pin, exceeding the absolute maximum voltage rating by 50%. And as they taught me in Electronics Technician A School in my Navy days, when you let the magic smoke out, the widget stops working.

And later

Finally, you may brick your Switch if you use USB-C to USB-A adapter cables. There are two variants of the cable; one with a 10K ohm resistor, and one with a 56K ohm resistor. This isn't directly dangerous to the Switch itself, but using a 10K ohm cable with a low-powered USB-A charger may overload it, damaging the charger—and a damaged charger could produce unexpected voltage on an output pin, which might in turn damage the Switch as well.

So if you're looking for a faster charge, I would do your homework and look for both a proper USB-C cable AND something that properly implements the USB-PD standard.