running bash pipe commands in background with & ampersand

You should put the & inside the (), if you want to run all the jobs in parallel in the background.

time for i in `ls /tmp/chunk*`; do
  (cat $i | tr ' ' '\n' | sort | uniq > /tmp/line${i:10} &)
done

You can include the & in braces:

time for i in `ls /tmp/chunk*`; do
  {(cat $i | tr ' ' '\n' | sort | uniq > /tmp/line${i:10}) &};
done