Why my battery slows down in speedy way

@addiselehu it is possible that this is a bad battery. Of course, you want to try a hard reset of your phone as well. Just make sure that you have backed your pictures and contacts etc. up. You will lose those in a hard reset.

If that did not work replace the battery. After that this sounds like a daughter board issue. Something like this one

First check your charger. Make sure it has proper output. Then check your charging cable. Check to make sure connectors are not bend etc. After that, check the charging port. Clean it of any debris etc, use a few puffs of air to clean it out. If none of this works, get a USB ammeter and see if your phone draws any amperage from the charger.

If that doesn't change anything, follow the Samsung flow chart and measure the components accordingly. The components are the ones the blue arrows are pointing to.

For any abnormality you find while you work on your phone, you can always add pictures to your Question. That way we can see what you see.

在已经存在的问题里加入图片

While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same problem as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.

Repair is War on Entropy!