The tcpdump is apparently buffering output when it writes to a pipe. It's not flushing output for each write, so the system will write the output in about 4k byte chunks. Your filter is limiting out put so you won't see anything until that filter has written enough output. Once it collects enough it will be written out in a chunk and you should see several lines emitted then.

Try to trigger DNS lookups many times and see what happens then.