Synchronous for multiple users or within the list itself? The items wouldn’t get scheduled instantly at the exact same time - Bubble queues them up (which you can view in Logs > Scheduler). Usually a list of items can be finished within seconds, but it really depends on the workflow and the length of the list.
You can run the Schedule API Workflow on List action on max of 20K items in a single search (Schedule API workflow on a list - does the list have a limit?)