BLE Uart,how to sent out faster after connected()

hello

NCS2.0.2/2.3.0, vscode

i add a fun as follow:
void PutToBleSend()
{
    struct uart_data_t *buf = k_malloc(sizeof(struct uart_data_t));
    if (buf) {
        sprintf(buf->data, "KEY-003");
        buf->len = strlen(buf->data);
         k_fifo_put(&fifo_uart_rx_data, buf);
    } else {
        printk("[log] No buffer\n");
        return;
    }
}
 
and call it in 
static void connected(struct bt_conn *conn, uint8_t err);
but bt_nus_send()  will return -128 in void ble_write_thread(void);
wait 2 second,  bt_nus_send() will send ok.
  
1.How can data be sent out faster after connected()?
2.How to creat a bounds.(test in nrf ToolBox uart)
  
 Best regards
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