bash while loop threading

You can start multiple processes, each calling the function check and wait for them to finish.

while read line 
do 
  ip=$line
  check &
done < $hosts
wait # wait for all child processes to finish

Whether this increases the speed depends on available processors and the function check's implementation. You have to ensure there's no data dependency in check between iterations.

Use GNU Parallel:

parallel check ::: $hosts

You can send tasks to the background by & If you intend to wait for all of them to finish you can use the wait command:

process_to_background &
echo Processing ...
wait
echo Done

You can get the pid of the given task started in the background if you want to wait for one (or few) specific tasks.

important_process_to_background &
important_pid=$!
while i in {1..10}; do
    less_important_process_to_background $i &
done

wait $important_pid
echo Important task finished

wait
echo All tasks finished

On note though: the background processes can mess up the output as they will run asynchronously. You might want to use a named pipe to collect the output from them.

edit

As asked in the comments there might be a need for limiting the background processes forked. In this case you can keep track of how many background processes you've started and communicate with them through a named pipe.

mkfifo tmp # creating named pipe

counter=0
while read ip
do
  if [ $counter -lt 10 ]; then # we are under the limit
    { check $ip; echo 'done' > tmp; } &
    let $[counter++];
  else
    read x < tmp # waiting for a process to finish
    { check $ip; echo 'done' > tmp; } &
  fi
done
cat /tmp > /dev/null # let all the background processes end

rm tmp # remove fifo