QuoteI'm not sure what the underlying issue is, but the workaround will be to redirect the output of sed to a new file and then move the new file to the original filename and perform the change in two steps not one.
I'll try it.
I don't think the Fritzbox is the problem. This works with all my other devices in the same way. When doing the same from my notebook it's the same "slow" fritzbox.