To determine the charging efficiency and speed of a power bank, you can start by checking its output current (measured in amperes or amps) and voltage (measured in volts). The higher the output current, the faster your device will charge. Typically, an output of 2A or above provides faster charging compared to 1A. Also, consider the power bank's capacity (measured in milliampere-hours or mAh) to understand how many times it can charge your device before needing a recharge itself. Furthermore, look for power banks with advanced technologies like Quick Charge or Power Delivery that support faster-charging speeds for compatible devices. Lastly, read user reviews and seek expert opinions to gauge real-world performance concerning charging efficiency and speed.