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I took traces with a monitor interface and noticed that the slower notebooks sent one acknowledgement frame for every data frame, not one Block Acknowledgement frame for many (64?) aggregated frames. The latter feature, "AMPDU", was switched off in the driver. In iwlwifi. By default. But only on some notebooks. m(

This link saved me:
https://askubuntu.com/questions/497395/14-04-wireless-throughput-issues