Cleaning can still help if it only slow charges (if you mean it used to be able to use high wattage ones).

Gunk prevents a strong connection, which can mess with the handshake. Charger will say, “yeah, I can fast charge, check out these amps!” but not all of it gets through and the case will decide the charger is a liar and just go with slow charging. Don’t assume that something getting through at all means the connection is fine because USB has fallback options when conditions are sub-optimal.