First thing I'd try is to remove all peripherals. External mouse/keyboard and drives etc. Power down and remove the AC power adaptor and battery. Press and hold the power button for 30 seconds or more. Refit battery and connect AC adaptor. See if that works. Otherwise I would send it back although it will be a hassle and takes time. Personally, with an issue like this, possibly the CPU is throttling down to save power as it's not getting a correct power supply reading, so the laptop 'lags', I wouldn't trust it and would certainly send it back under warranty.
Xiaomi Mi 8 Review