I am using a rather simple sample code to write into and read from analog IOs:

    t1 = time.time() # start time in s
    d=np.empty([10000])

    for i in range(10000):

        adcdac.set_dac_voltage(1, 1.5*i/10000) #set the voltage on channel 1 
        d[i]=adcdac.read_adc_voltage(1, 0)

    t2 = time.time() # end time in s
    dt = t2 - t1 # time delta

The measured time is approx 1 s, which means that I can transfer 10000 IO pairs (one write and one read) in 1 second. Read only takes 0.62 s for 10.000 reads. So this is all I can achieve, which is far off the 70 kHz communicated somewhere else?

The performance problems you are having are coming from using Python.

Python is an interpreted language which has a far lower performance than compiled programming languages like C or C++.
To get the most performance out of the ADC DAC Pi I would recommend using a compiled programming language like C.

To give you an idea of the performance difference I rewrote your program using the C version of our ADC DAC Pi library and ran it on a Raspberry Pi B 3+. The C program takes approximately 22.3 milliseconds to complete the same 10000 loops as your python program.

The code for the C program is shown below. You can download the ADCDAC Pi C library from our Github repository.

/*
 * test.c
 * compile with "gcc ABE_ADCDACPi.c test.c -o test"
 * run with "./test"
 */

#include <stdio.h>
#include <stdint.h>
#include <sys/time.h>
#include <time.h>
#include <unistd.h>

#include "ABE_ADCDACPi.h"

void clearscreen ()
{
    printf("\033[2J\033[1;1H");
}

int main(int argc, char **argv){
	setvbuf (stdout, NULL, _IONBF, 0); // needed to print to the command line

	struct timeval t1, t2;
	double dt;

	double d[10000];

    set_dac_gain(1);

	// start timer
	gettimeofday(&t1, NULL);

	int i;

	for (i = 0; i <= 10000; i++){
		set_dac_voltage(1, 1.5*i/10000);
        d[i] = read_adc_voltage(1, 0);
	}

	// stop timer
	gettimeofday(&t2, NULL);

	// compute and print the elapsed time in millisec
	dt = (t2.tv_sec - t1.tv_sec) * 1000.0;      // sec to ms
	dt += (t2.tv_usec - t1.tv_usec) / 1000.0;   // us to ms

    
    printf("%G",dt);

	return (0);
}