nRF52 SDK 15 iOS -> low data rate

Using nRF52832 w/ SDK 15.2 as peripheral connecting to an iOS 12.2 central (iPhone XS Max).

IDE: SEGGER Embedded Studio for ARM, Release 4.10a

The max data rate I can achieve is ~1250 Bytes per second using sd_ble_gatts_hvx notifications. Once I attempt to go over this, say ~2kBytes/sec then within a few hundred packets NRF_ERROR_RESOURCES is returned ... which I buffer and retry sending after BLE_GATTS_EVT_HVN_TX_COMPLETE is returned. The retry buffer has size 10 packets. Once the buffer is full then it dumps/loses the data and keeps going til the next overflow.

I've read several Nordic forums claiming much higher data rates (~775kbits 3 years ago :) and I've followed the parameters they suggested but still hitting the wall.

Current parameters:

- Data Length Extension: 251

- Connection Event Extension: Enabled via sd_ble_opt_set(BLE_COMMON_OPT_CONN_EVT_EXT, &optionsConnExt);

- #define NRF_SDH_BLE_GATT_MAX_MTU_SIZE 247 (iOS negotiates to 185)

- My payload size: (NRF_SDH_BLE_GATT_MAX_MTU_SIZE - 3)

- Tested with following MIN_CONNECTION_INTERVAL:
   * #define MIN_CONNECTION_INTERVAL (uint16_t) MSEC_TO_UNITS(7.5, UNIT_1_25_MS)
   * #define MIN_CONNECTION_INTERVAL (uint16_t) MSEC_TO_UNITS(30, UNIT_1_25_MS)
   * #define MIN_CONNECTION_INTERVAL (uint16_t) MSEC_TO_UNITS(200, UNIT_1_25_MS)
   * #define MIN_CONNECTION_INTERVAL (uint16_t) MSEC_TO_UNITS(550, UNIT_1_25_MS)

- #define MAX_CONNECTION_INTERVAL (uint16_t) MSEC_TO_UNITS(600, UNIT_1_25_MS)
- #define SLAVE_LATENCY 0 // Slave latency in terms of connection events.
- #define SUPERVISION_TIMEOUT (uint16_t) MSEC_TO_UNITS(4000, UNIT_10_MS) // Supervision time-out in units of 10 milliseconds.


- Other values: max_rx_octets: 251, max_tx_octets: 251, max_rx_time: 1364, max_tx_time: 1364

Any suggestions on how to get close to 100kBytes/second?