If you backup your iphone (essential) and then restore it and update to the latest version, and it's still not showing the icon, then we have determined that this is a hardware issue and not a software issue.
Do you have Apple Care?
Book an appointment via contacting apple support, you can do this on live chat as soon as they determine that your phone is acting up.
Do you still have your warranty?
Book an appointment via contacting apple support, you can do this on live chat as soon as they determine that your phone is acting up.
Don't have any of these?
I would still contact support. They can book an apointment but you will have to pay a fee for not having a warranty, and then a stupidly expensive repair cost
Are you sure the phone is charging slowly? 3 Hours using wall charging and 4 hours using a usb in a laptop/tv/something not in a plug. (Around 3-4 hours maybe 5 depending on the age of the phone)
Keep in mind the phone will 'fast charge' from 0% to 80%, then 'trickle charge' more slowly from 80% to 100%.