My OnePlus 7T Pro started charging very slowly. With only Firefox open, it could not increase charge whilst streaming a youtube video; merely stop it from dropping.
It wasn’t cables or chargers.
Anyway, the solution was to close excess, hidden Firefox tabs. Not sure if one/more particular tabs were to blame, or just total, but anyway, if you are having this issue, consider parasitic loads in this form.
Regain your privacy! Adopt /e/OS the deGoogled mobile OS and online services