OK, so I have set up a test package that calls master1 4 times in a row while resetting things like target database tables to their original state in between calls. These are the results:

Master1 call1: 04:40

Master1 call2: 05:42 = increase of 22%

Master1 call3: 06:44 = increase of 44%

Master1 call4: 08:29 = increase of 82%

There has to be some sort of resource problem