Sorry brain entirely not working and I copied the wrong number from an entirely different mail about something else.
I was wondering what the theoretical overhead is - largest packet is 376us, full-sized non-ecrypted packets are 344us, smallest is 80us, you need one packet in each direction consecutively so that's 424us and that sends 20 bytes of data one-way. That leads to a theoretical max of 47kB/s. The requirement to end the connection event in time to let the radios switch cuts that down and it's further cut down by various implementations restricting the number of packets per connection interval and the min spacing between them.
It's not very fast, but it wasn't designed to be that fast.
