Hi @enrico,
If you check out within the Tobii Core SDK Interaction API, you will see the we provide the eye position data is both normalised format (as in Stream API) but only in absolute coordinates with the origin relative to the screen being tracked.
You can check out this data stream within the “Interaction_Streams_103” sample application which can be downloaded on the Core SDK sample page @ https://github.com/Tobii/CoreSDK
The data (including Z acis) is lightly filtered for stability, however reaction time should not be overly slow.. can you replicate the slowness on other users?