Samsung a51 rubbish “fast charge”

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Hi, just recently bought a Samsung a51 and, moving from a oneplus3 (4 year old tech), the fast charge is just rubbish. It says fast charge when charging, but measuring using a wall usage monitor, it's only drawing a paltry 5w. It's meant to be 15w, but on accubattery it's only charging at max 1200mA when oneplus3 (rated at 18w) used to do 3500mA.

anyone else noticed this, going to return tomorrow if a restart doesn't fix it. Feels like taking a big step back on a small rated difference

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So it seems that this phone only charges at 12w max when the screen is off. When running the phone it goes to a stupidly low 5w. Problem is the screensaver also seems to prevent it from "fast charging", so most of the time it's just going along at 2001 speeds. 

is this an issue with this handset, or are they all this bad?