Ble_Central connect to periphral spend a long time

Dear Nordic

I use nRF5 SDK BLE_Central / uart to connect our custom peripherals by address, and I found that it seem need to 4~5 seconds to finish connecting 

Here is the log. Is there any way to improve the connection speed?

Thanks

 [00:00:05.637,329] <debug> ble_scan: Connecting
 
 [00:00:05.637,329] <debug> ble_scan: Connection status: 0
 
 [00:00:06.145,446] <debug> nrf_ble_gatt: Updating data length to 251 on connection 0x0.
 
 [00:00:06.145,507] <info> app: Connecting to target E11xxxxxxx
 
 [00:00:06.145,568] <debug> nrf_ble_gq: Registering connection handle: 0x0000
 
 [00:00:06.145,629] <debug> app: conn set to 4dBm successfully
 
 [00:00:06.326,904] <debug> nrf_ble_gatt: Data length updated to 27 on connection 0x0.
 
 [00:00:06.326,904] <debug> nrf_ble_gatt: max_rx_octets: 27
 
 [00:00:06.326,904] <debug> nrf_ble_gatt: max_tx_octets: 27
 
 [00:00:06.326,904] <debug> nrf_ble_gatt: max_rx_time: 328
 
 [00:00:06.326,965] <debug> nrf_ble_gatt: max_tx_time: 328
 
 [00:00:07.345,642] <debug> app: Requesting to update ATT MTU to 247 bytes on connection 0x0.
 
 [00:00:07.586,975] <debug> nrf_ble_gatt: ATT MTU updated to 247 bytes on connection 0x0 (response).
 
 [00:00:07.586,975] <info> app: ATT MTU exchange completed.
 
 [00:00:07.586,975] <info> app: Ble NUS max data length set to 0xF4(244)
 
 [00:00:07.586,975] <debug> nrf_ble_gq: Processing the request queue...
 
 [00:00:07.845,825] <debug> ble_db_disc: Starting discovery of service with UUID 0xAE30 on connection handle 0x0.
 
 [00:00:07.845,825] <debug> nrf_ble_gq: Adding item to the request queue
 
 [00:00:07.845,825] <debug> nrf_ble_gq: GATTC Primary Services Discovery Request
 
 [00:00:07.845,886] <debug> nrf_ble_gq: SD GATT procedure (2) succeeded on connection handle: 0.
 
 [00:00:08.846,984] <debug> ble_db_disc: Found service UUID 0xAE30.
 
 [00:00:08.846,984] <debug> nrf_ble_gq: Adding item to the request queue
 
 [00:00:08.846,984] <debug> nrf_ble_gq: GATTC Characteristic Discovery Request
 
 [00:00:08.847,045] <debug> nrf_ble_gq: SD GATT procedure (3) succeeded on connection handle: 0.
 
 [00:00:08.847,045] <debug> nrf_ble_gq: Processing the request queue...
 
 [00:00:08.967,773] <debug> nrf_ble_gq: Adding item to the request queue
 
 [00:00:08.967,773] <debug> nrf_ble_gq: GATTC Characteristic Descriptor Request
 
 [00:00:08.967,834] <debug> nrf_ble_gq: SD GATT procedure (4) succeeded on connection handle: 0.
 
 [00:00:08.967,834] <debug> nrf_ble_gq: Processing the request queue...
 
 [00:00:09.086,975] <debug> nrf_ble_gq: Adding item to the request queue
 
 [00:00:09.086,975] <debug> nrf_ble_gq: GATTC Characteristic Descriptor Request
 
 [00:00:09.087,036] <debug> nrf_ble_gq: SD GATT procedure (4) succeeded on connection handle: 0.
 
 [00:00:09.087,036] <debug> nrf_ble_gq: Processing the request queue...
 
 [00:00:09.206,970] <debug> nrf_ble_gq: Adding item to the request queue
 
 [00:00:09.207,031] <debug> nrf_ble_gq: GATTC Characteristic Descriptor Request
 
 [00:00:09.207,031] <debug> nrf_ble_gq: SD GATT procedure (4) succeeded on connection handle: 0.
 
 [00:00:09.207,092] <debug> nrf_ble_gq: Processing the request queue...
 
 [00:00:09.326,965] <debug> ble_db_disc: Discovery of service with UUID 0xAE30 completed with success on connection handle 0x0.
 
 [00:00:09.327,026] <info> app: Discovery complete.
 
 [00:00:09.327,026] <debug> nrf_ble_gq: Adding item to the request queue
 
 [00:00:09.327,026] <debug> nrf_ble_gq: GATTC Write Request
 
 [00:00:09.327,087] <debug> nrf_ble_gq: SD GATT procedure (1) succeeded on connection handle: 0.
 
 [00:00:09.327,087] <debug> nrf_ble_gq: Adding item to the request queue
 
 [00:00:09.327,148] <debug> nrf_ble_gq: GATTC Write Request
 
 [00:00:09.327,148] <debug> nrf_ble_gq: SD is currently busy. The GATT request procedure will be attempted                       again later.
 
 [00:00:09.327,148] <debug> nrf_ble_gq: Pointer to allocated memory block: 0x20009798.
 
 [00:00:09.327,148] <debug> nrf_ble_gq: Processing the request queue...
 
 [00:00:09.327,209] <debug> nrf_ble_gq: GATTC Write Request
 
 [00:00:09.327,209] <debug> nrf_ble_gq: SD is currently busy. The GATT request procedure will be attempted                           again later.
 
 [00:00:09.327,209] <info> app: Connected to device with Nordic UART Service.
 
 [00:00:09.327,514] <debug> nrf_ble_gq: Processing the request queue...
 
 [00:00:09.327,514] <debug> nrf_ble_gq: GATTC Write Request
 
 [00:00:09.327,514] <debug> nrf_ble_gq: SD is currently busy. The GATT request procedure will be attempted                           again later.
 
 [00:00:09.327,575] <debug> nrf_ble_gq: Processing the request queue...
 
 [00:00:09.327,575] <debug> nrf_ble_gq: GATTC Write Request
 
 [00:00:09.327,575] <debug> nrf_ble_gq: Pointer to freed memory block: 0x20009798.
 
 [00:00:09.327,575] <debug> nrf_ble_gq: SD GATT procedure (1) succeeded on connection handle: 0.
 
 [00:00:09.327,575] <debug> nrf_ble_gq: Processing the request queue...

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