The "best performance" setting is recommended if you want to play games or are doing video editing, to ensure you fully utilize the potential of the onboard hardware of the Surface Book 2. The Verge points out some data about the power consumption and the maximum power delivery capacity.
It appears that the Surface Book 2 has been designed to supply 95 watts of power from the charger to the device, which isn’t enough to run the processor, graphics card, and all other hardware components at max. The processor alone draws 25 watts in high-power mode, and will even burst to 35 watts. Microsoft’s Nvidia GTX 1060 variant draws between 70 and 80 watts, bringing the total to 105 watts at peak. Microsoft works around this by aggressively throttling the Nvidia chip during games at “better performance” and “best battery” settings.Overall, it does't appear this is problematic. The maximum peak power consumption of the device is about 105W, and the power delivery system supports reloading of up to 95W. The Verge crunched the numbers and said the battery drainage occurs at a rate of at least 10 percent per hour, depending on the game and load. This means you can game many hours at full performance before you fully drain the battery. So not a huge issue, but definitely funny.