Hello! Many things can cause slow charging. It could be the applications that are continuously running in the background in which you can go into the settings and force stop those.
We would recommend using the original Samsung charger that came with your phone, Third party chargers could definitely cause the symptoms you're experiencing.
If you're just in a pinch and need a fast charge up we recommend using the adaptive fast charging to charge the phone. Check out the following link https://www.samsung.com/us/support/troubleshooting/TSG01001386/
Here's also some very helpful resources for extended your battery life:
I hope you find this helpful!