Gaze data from a Tobii EyeTracker 5 typically has a latency of ~10-16ms. If you compare the timestamp in the gaze samples with the system clock you can easily compute the latency of each gaze sample. The timestamp in the gaze sample represents when the eyetracking sensor captured the images of the eye. If you are using the streamengine api, you can compare the timestamp with the value you get from tobii_system_clock.