I did something similar during a cardiac stress test, with a Fitbit Versa. My best conclusion was that the Fitbit is at least +/-5 BPM off and it lagged behind

I did something similar during a cardiac stress test, with a Fitbit Versa. My best conclusion was that the Fitbit is at least +/-5 BPM off and it lagged behind the hospital equipment by roughly 15 seconds (judging by heart rate peaks).