Here are the latency measurements taken by one of our students. Images reflect the azimuthal component of the head and eye during vestibulo-ocular reflex (while fixating a stable object and rotating one's head about the vertical axis). The first figure shows the original signals - deg/s over time. The second shows the results of the cross correlation, and the third shows a zoomed in look at the signals before/after adjustment for the measured latency.
The two signals were maximally correlated with 83 ms of shift, meaning that there was 83 ms of latency in the eye tracker relative to the head tracker. Vive head tracking is typically 22-33 ms absolute latency, so that puts the ET above 100 ms of absolute latency.
We have only measured this once. Really, we should measure a few more times, with different people. We could also learn more by testing for possible effects of rotational velocity, and GPU load on the amount of latency.
An effect of the former (head velocity) would indicate that this latency results form a dynamic filter. This seems unlikely, because if it were, they would just have us turn it off.
It's more likely attributed to the core functions of the eye tracking pipeline, such as pupil segmentation in the 2D imagery, or gaze mapping (which converts from 2D pupil centroids to 3D gaze vectors). If so, I would expect the latency to increase with GPU load.
Something to test, anyhow. Or, HTC could jump in and say a bit more...