In your code you change the values and type of Werte 4 times. That seems a bit excessive. I'd try something like this:
1 while true
2 tic
3 data = importdata('C:\Users\Hüs\Documents\MATLAB\Datalogger\KlimaLoggProAuto.dat1');
4 alle = [data(8,1);data(11,1);data(21,1);data(24,1);data(34,1);data(37,1);data(47,1);data(50,1);data(60,1);data(63,1);data(73,1);data(76,1);data(86,1);data(89,1);data(99,1);data(102,1)];
5 string(alle);
6 a = split(alle,'"');
7 % Overwritten on the very next line: Werte=zeros(1,17); %preallocated value....might be unused.
8 Werte = a(:,2)';
9 Zeit = datetime('now');
10 Zeit = string(Zeit);
11 % Seems skippable: Werte=[Zeit Werte]; %Consider preallocating for speed.
12 ZeitWerte_string = cellstr([Zeit,Werte]);
13 [succes,message] = xlsappend('Datalogger.xlsx',ZeitWerte_string,1);
14 pause(600-toc)
15 end
HTH
