Apps Script is inherently synchronous – you cant do lots of things at once. Primarily to get round the execution limit, I did a science project using triggers for parallel processing but the triggers are fairly unpredictable in when they get round to executing.
The answer is YES and YES and YES!
There’s a new and improved version of this over on Orchestration of Apps Scripts – parallel threads and defeating quotas
Here’s a quick 15 second video of simple map/reduce exercise of 3 map tasks in parallel followed by a reduce of the results.