How to execute multiple instance subprocess in parallel?

Issue

There is a process where rest service is being called from multi instance node, it looks like this:

start node->script node -> multi instance (Here we have start node->REST task->end) -> end node

It is requested that MI task executes in parallel. However when placed Thread.sleep is placed in one REST call, MI stopped execution of remaining rest until sleep got over which means it's not a parallel execution but sequential.