it takes 750ms for one to calculate the temp and I have a few
I hope you realize that that conversion will take about 750ms for all together. You don't have to let every sensor 'convert' one-by-one. At least without parasitic power that is. While not with parasitic power you can also poll the dataline line to check when the conversion is actually done. My string of 10 18x20 sensors takes about 600ms to convert the temperature.
Regards,
Jeroen