Frequency of Notifications with Android BLE Library

I am working on a derivative of the Glucose sensor code from nRF Toolbox. The sensor peripheral accumulates a number of readings. The central  sends a command to the Record Access Control Point characteristic asking for the readings. The peripheral sends the requested data as a sequence of Notifications from the Glucose Measurement Characteristic, then sends a status message as an Indication from the Record Access Control Point characteristic.

For a small number of readings this works as expected: I see the Notifications arriving at the central (in the Notification callback code onDataReceived() ).

However, I find that when the number of characteristics exceed about 6, the central stops receiving the Notifications. The peripheral hangs up as it sends the Notification AFTER the last one received by the central.

If I insert a delay of about 8ms on my device, to throttle the Notification stream, then it starts working again. I am doing some work on the incoming Notification data (checking validity) then add it to an Array List for later processing, so probably not spending a huge amount of time.

The question is whether there are limitations at the central on the rate at which it can receive Notifications. Are Notifications queued by the OS? Do I need to do somewthing with threads? etc.

I get the same result with two tablets running Android 7.