Thanks a lot macropod!
The first advice reall helped to speed things up.
When I inserted the "If i Mod 100 = 0 Then DoEvents" the duration did not change.

However it still takes a really long time to compute larger sets of data.
For example I tried this for 500 observations and it took 25 seconds, but with 1000 observations it took 2:40 minutes. So the time grows exponentially. Apparently Excel has problems with larger samples.
Do you think it might make sense to conduct the goal seek function in consecutive sheets to work with smaller numbers of observations each time?
And if yes how would the loop for this have to look like?

Thanks in advance!