First question
How much power does the laptop actually expect?
Check the original charger label or the manufacturer's specification. A laptop designed around a higher-wattage adapter can accept USB-C charging and still warn that the connected source is slower than the preferred adapter. The important comparison is the laptop requirement versus the power available from the exact charger port you are using.
Step 01
Disconnect every other charger port.
A multi-port charger might be marketed as “100W” because 100W is its total power budget or its maximum on one port under certain conditions. Once a phone, tablet, or second laptop is connected, many chargers redistribute that budget. Test the laptop as the only connected device.
Step 02
Check the output table printed on the charger.
Look for the tiny voltage/current table or the manufacturer's port-allocation chart. You want the maximum output of the specific USB-C port by itself and the output assigned to that same port when multiple ports are in use. Do not rely on the large “100W” number on the front of the charger.
Step 03
Use a cable that can carry the required current.
Microsoft lists an underpowered or incompatible cable as a cause of the Windows slow-charger warning. If the laptop expects more than 60W, use a cable explicitly designed for the higher USB Power Delivery current level. During diagnosis, remove extension cables and magnetic adapters.
Step 04
Use the laptop's actual charging port.
Some laptops have multiple USB-C connectors with different capabilities. Microsoft specifically lists using the wrong USB-C port as a possible reason for charging problems. Check the manual and connect the charger to the port documented for charging or USB Power Delivery.
Step 05
Bypass the dock during the test.
If the charger is connected to a dock and the dock powers the laptop, the dock becomes part of the power path. The dock may reserve power for itself and its downstream devices, or it may have a lower laptop-charging limit than the charger connected to it. Plug the charger directly into the laptop. If the warning goes away, the dock path is the constraint.
Decision table
What the result tells you.
The charger is power-sharing. Use the preferred port combination or a charger with more simultaneous headroom.
The old cable was the limit. Replace the cable, not the charger.
The dock's charging limit or reserved power is the bottleneck.
Compare the laptop's required input with the charger port's actual supported output.
Sources