BLE Timing Variation While Streaming Live Microphone Data

Question

Hi nordic, 
 I am streaming live microphone PCM data over BLE. The microphone data is continuously captured and stored in a buffer. Whenever 12 KB of audio data is available, I send that data over BLE to the connected device. 
 However, I am observing that sending the data over BLE takes a large and inconsistent amount of time. 
 For example, I measured the time taken by the send_data_chunk() function, which sends the data over BLE. the api i am using is : 
 
 // Send data in 244-byte chunks // ptr_buffer contains the live data 
 // no_of_bytes is 12288 
 // creating the chunk of 244 among 12288 , and sending over ble. 
 void send_data_chunk( struct bt_conn *conn, uint8_t *ptr_buffer, uint16_t no_of_bytes) 
 { 
 static uint8_t first_time = 0 ; 
 if (!conn) 
 { 
 printk( "Error: No BLE connection.
" ); 
 } 
 
 // Calculate chunk size based on negotiated MTU 
 uint16_t chunk_size = 247 - 3 ; // MTU - ATT Header (3 bytes) //current_mtu 
 if (chunk_size == 0 ) 
 { 
 printk( "Error: Invalid chunk size (MTU too small).
" ); 
 } 
 
 uint16_t data_sent = 0 ; 
 int err = 0 ; 
 uint32_t start_time = k_uptime_get_32(); 
 
 while (data_sent < no_of_bytes) 
 { 
 uint16_t bytes_to_send = MIN (chunk_size, no_of_bytes - data_sent); 
 
 err = bt_gatt_notify(conn, &custom_service.attrs[DATA_NOTIFY_INDEX], 
 ptr_buffer + data_sent, bytes_to_send); 
 
 if (err) 
 { 
 if (err == - ENOMEM || err == - EAGAIN ) 
 { 
 printk( "BLE notification queue full or busy (err %d). Retrying...
" , err); 
 k_sleep( K_MSEC ( 1 )); // Sleep briefly to allow BLE stack to process 
 continue ; // Try sending the same chunk again 
 } 
 else 
 { 
 printk( "Notification failed (err %d) for chunk at offset %u.
" , err, data_sent); 
 // return err; // Other error, stop sending this block 
 } 
 } 
 data_sent += bytes_to_send; 
 } 
 
 uint32_t end_time = k_uptime_get_32(); 
 uint32_t diff_time = end_time - start_time; 
 if (diff_time >= 10 ) 
 { 
 printk( "Total send_data_chunk time for %u bytes: %u ms
" , no_of_bytes, end_time - start_time); 
 } 
 if (first_time == 0 ) 
 { 
 first_time = 1 ; 
 printk( "Total send_data_chunk time for %u bytes: %u ms
" , no_of_bytes, end_time - start_time); 
 } 
 
 } and the prj configurations i am using

CONFIG_BT =y 
 CONFIG_BT_PERIPHERAL =y 
 CONFIG_BT_DEVICE_NAME = "FYZKS_WDS" 
 CONFIG_BT_GATT_CLIENT =y 
 CONFIG_BT_ZEPHYR_NUS =y 
 
 CONFIG_BT_USER_DATA_LEN_UPDATE =y 
 CONFIG_BT_CTLR_DATA_LENGTH_MAX =251 
 CONFIG_BT_BUF_ACL_RX_SIZE =251 
 CONFIG_BT_BUF_ACL_TX_SIZE =251 
 CONFIG_BT_L2CAP_TX_MTU =247 
 CONFIG_BT_USER_PHY_UPDATE =y 
 CONFIG_BT_GAP_AUTO_UPDATE_CONN_PARAMS =n 
 
 # Connection interval settings 
 CONFIG_BT_PERIPHERAL_PREF_MIN_INT =6 
 CONFIG_BT_PERIPHERAL_PREF_MAX_INT =8 
 CONFIG_BT_PERIPHERAL_PREF_LATENCY =0 
 CONFIG_BT_PERIPHERAL_PREF_TIMEOUT =400 
 CONFIG_BT_CTLR_TX_PWR_PLUS_4 =y

Below are the timing logs (internal debugging messages removed for clarity): Total send_data_chunk time for 12288 bytes: 873 ms Total send_data_chunk time for 12288 bytes: 887 ms Total send_data_chunk time for 12288 bytes: 896 ms Total send_data_chunk time for 12288 bytes: 888 ms Total send_data_chunk time for 12288 bytes: 903 ms 
 From these logs, I observe:

Sending 12 KB of data takes around 880–900 ms

the timing is not constant . Sometimes the transmission completes in around 800 ms , while at other times it takes up to 1800 ms for similar amounts of data.(when flashed the same code again repeatedly whether we are achieving the same time or not )