Slave labor is most efficient when it comes to non-skilled, hard work. Mining, agriculture, sex (where it still survives even in the Western world), where the output is easily checked and counted.

When it comes to anything sophisticated done by qualified people, like "making advanced tools for the Führer", the options for subtle sabotage are there and pissed-off people will use them.

In general, German occupation authorities had better results when they actually paid the workers and gave them vacation vouchers. But of course the racial theories got in the way, as it was unthinkable to treat, say, Jews as normal employees.