I need advice for building a bandwidth-optimized link over BLE

Thanks everyone for your inputs!

I'd advise this too. It's just super easy, and Espruino handles this more efficiently than a custom BLE service. If you're looking at x/y/z every 500ms bandwidth isn't going to be much of an issue for you at all though :)

For one Puck.js it seems definitely to be the best way to go, however I would like to have let's say 5 or more Pucks running simultaneously, and 500ms is a minimum. I did not explain well what I was trying to do: I would like to build a 3d motion sensing platform, a bit like what they use in film-making for motion capture. I am unsure of the precision I will get but it is worth a try.

At first I was thinking of using a Raspberry Pi as a central device, able to establish several connections simultaneously, but I would also love to try and evaluate the performance of an Espruino device as a central device, if the feature is being added to the firmware at a given time ;-)
Currently, I think that the EspruinoHub (MQTT) could help me implementing the central device.