HC06 receive data speed

I have a problem about receiving data with mega2560 and HC06.
I use android phone to send data.
I receive 29k byte data using 1 mins.
(I send 1000 byte and delay 2 sec.Arduino use 2 sec receiving data.)
If I delay less than 2 sec,the data won't be complete.
how can I improve speed?
Next,I will send 1M~4M byte to arduino and save data in SD card.

#include <SoftwareSerial.h>

#include <Wire.h>

 
#define MAX_BTCMDLEN 1000


SoftwareSerial BTSerial(10,11); // Arduino RX/TX



void setup() {

    Serial.begin(9600);  

    BTSerial.begin(9600); 
    Serial.println("GO");
    Serial.println();
}

 

void loop() {

    char str[MAX_BTCMDLEN];

    int insize, ii;  

    int tick=0;
    
    byte cmd[MAX_BTCMDLEN]; 
  
    int len = 0; // received command length
    
    while ( tick<MAX_BTCMDLEN ) { 
       
        if ( (insize=(BTSerial.available()))>0 ){ 
            for ( ii=0; ii<insize; ii++ ){

                cmd[(len++)%MAX_BTCMDLEN]=char(BTSerial.read());

            }

        } else {

            tick++;

        }

    }

    if ( len ) {

     
        
        if(len==MAX_BTCMDLEN){    
          BTSerial.write(":OK");
          Serial.print("----------");
            Serial.println(len);
            Serial.println("");
        }
        else
          BTSerial.write(":NO");

            
            for(int i=0;i<len;i++)
                cmd[i]=0; 
         
    }        
        

   

    len = 0;


}

how can I improve speed?

    BTSerial.begin(9600);

Do you have a clue what this does? Some time at the reference page is in order.

    if ( len ) {

If len what? Married his high-school sweetheart?

BTSerial.begin(9600);
I know the meaning of '9600'.(9600 bits per sec)

if I want to improve receiving speed,I set baud to '115200'?

 if ( len ) {

If len is bigger than 0,I know the arduino receive data.

AKAI:
if I want to improve receiving speed,I set baud to '115200'?

Yes, it will be as dramatic as you would expect but, while the HC-06 will be fine at 115200, you will need to get off software serial and use hardware serial, pins 0,1. I bet you never had a good reason for using software serial anyway, and now you have a very good reason for not using it .