Why does my laptop power adapter need to warm up before it works properly?

That behavior strongly suggests a failing component inside the adapter, not normal operation. Aging electrolytic capacitors are a common suspect: their electrical characteristics can change with temperature, so the supply may become more stable after it has warmed up. A cracked solder joint or another temperature-sensitive fault is also possible. Since this is a high-power mains-powered supply, it is not safe to open and troubleshoot casually. Replacing it with a compatible 240 W adapter is the practical fix.

The gradual-load test doesn’t necessarily mean the charger is safely ramping its current. The adapter may be repeatedly entering and leaving overcurrent, undervoltage, or thermal protection as the laptop load changes. A proper voltage measurement under load would help confirm whether the 20 V output is collapsing, but measuring inside or modifying the brick can expose you to dangerous mains voltage. Use a genuine replacement with the same voltage, connector, polarity, and sufficient wattage rather than continuing to warm up the faulty one.

Thermal expansion could contribute if there’s a bad solder connection, but the more likely explanation is an aging capacitor or another component whose performance improves when warm. At 240 W, unstable output can also stress the laptop and the adapter. If the brick is buzzing, overheating, swelling, smelling unusual, or showing damaged cables, stop using it immediately and replace it.