Solved! overcharge your laptop battery

1. You can play without being plugged in. But a loaded CPU + GPU typically exceeds the current draw that the battery is capable of providing. So the laptop will automatically clock down the CPU and GPU to keep power consumption within the ability of the laptop to provide. So framerates will be substantially worse when playing on battery (expect around half). Also, battery life will be pathetic. You'll be lucky to get more than 1 hour.

2. Whether leaving the laptop plugged in all the time is good or bad depends on how the laptop manufacturer has it set up. Some deliberately underrate the battery's capacity. So when Windows reports 100% charge, it's actually at around 85% charge. It's the charges to 100% (and discharges to 0%) which harm the battery the most. So limiting charges to 85% in this way helps extend battery longevity. (EVs do the same thing - they're typically limited to operate between 20% and 80% charge, displaying those as empty and full charge)

Other manufacturers give you a utility to manually set the max battery charge state. So you can, for example, set the laptop to stop charging when it hits 80%, and not recharge until it self-drains to 70%. Lenovo takes this approach with the Thinkpads. It's one of the functions in the Lenovo Vantage toolbar. I do not know if Lenovo includes this function in its non-Thinkpad laptops.