Why my phone is charging too slow?

@hamzamujahid could be but it can also be a few other thing. I would start off by replacing the battery. If that does not improve things, Replace the charging board. Since iFixit does not yet have a guide for this, use something like this video Replacement parts are readily available at many online stores.

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 a device with the same issues 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!