Why process substitution is faster than pipe in this case? [duplicate]

Question

I am wondering why

ls -1 | 
while read file; do 
     echo $file; tail -n 100 $file > >(sleep 1 && cat > $file)
done  

is faster than

ls -1 | 
while read file; do 
    echo $file; tail -n 100 $file | (sleep 1 && cat > $file)
done   

?

If there are 100 files in a directory then:

• the second command takes almost 100 seconds to process
• the first command is processed almost immediately.

Answer 1

When you use a pipeline, the shell runs each command in the pipeline concurrently, and waits for all of them to finish before going to the next command. This is explained in the documentation:

If the pipeline is not executed asynchronously (see Lists), the shell waits for all commands in the pipeline to complete.

When the above refers to executing the pipeline asynchronously, it's talking about running the whole pipeline in the background with &.

When you use process substitution the shell doesn't wait for it to complete. The documentation simply says:

The process list is run asynchronously, and its input or output appears as a filename.