clEnqueueReadBuffer sometimes too slow(2second)

clEnqueueReadBuffer sometimes too slow(2second)

Hello, I'm making real-time application but sometimes clEnqueueReadBuffer is too slow.
I tested clEnqueueReadBuffer with attached code. But I don't know why it is slow. Please help.

this is my environment.

OS: Windows 10 Pro 64 bits
CPU: Intel(R) Core(TM) i7-7700 CPU @ 3.60GHz
GPU : Intel(R) HD Graphics 630
OpenCL: 1.2 version.
Intel OpenCL SDK:
Version=6.3.0.1904
InternalVersion=dkdnfngdfkjndfkjgndfndfgk
Visual Studio Professional 2015.

below is my test code. full project is attached.

	cl_mem					d_buf;
	unsigned char *			h_in;
	unsigned char *			h_out;
	int						byte;

	unsigned long			tick_start;
	unsigned long			tick_end;
	int						idx;


	// initialize
	byte	= 4096;

	h_in	= new	unsigned char[4096];
	h_out	= new	unsigned char[4096];
	d_buf	= clCreateBuffer( ocl.context, CL_MEM_READ_WRITE, byte, NULL, &err );

	::memset( h_in, 0, byte );
	err = clEnqueueWriteBuffer( ocl.commandQueue, d_buf, CL_TRUE, 0, byte, h_in, 0,	NULL,NULL );
	if ( CL_SUCCESS != err ) {
		printf( "WriteError %d \r\n", err );
		DebugBreak();
	}


	// main loop
	for ( idx = 0 ; idx <= 500000 ; idx ++ ) {
		tick_start = ::GetTickCount();

		err = clEnqueueReadBuffer( ocl.commandQueue, d_buf,	CL_TRUE, 0, byte, h_out, 0,	NULL, NULL );
		if ( CL_SUCCESS != err ) {
			printf( "ReadError %d \r\n", err );
			DebugBreak();
		}

		tick_end = ::GetTickCount();

		// for check progress
		if ( idx %10000 == 0 ) {
			printf( "idx: %d \r\n", idx );
		}

		// for check large delay
		if ( tick_end - tick_start > 100 ) {
			printf( "idx: %d, Elapsed: %d ms \r\n", idx,  (int)(tick_end - tick_start));
		}
	}

	// rlease memory
	clReleaseMemObject( d_buf );
	delete [] h_in;
	delete [] h_out;