Hello Johannes,
As I mentioned you should be sure the 'outMerge' is feature layer, but if you have a feature layer with high number of records, calling arcpy inside a for loop in each iteration is not efficient method.
If you try the both solutions in your feature layer data, you will see the differences.
I just tried in a feature layer with 257 records,
first one took 7.94 seconds.
second one took 0.18 seconds.
Anyway, I am so happy you find your solution,
Best ragrads
Amir Sarrafzadeh Arasi