For http calls the core-service (dms-service) uses the default thread pool. This pool is by default limited to 10 concurrent threads. Any additional call will wait until a thread from the pool becomes available. This can be changed by modifying the standalone-full-ha.xml in the standalone/configuration folder. But in most cases it will not help to archive more performance to do so. The hard limitation is set by physical cpu's in the system and by database speed. To use more concurrent threads won't help in most cases.
For the creation of many objects it will help to use the DmsBatchService.createObjects endpoints. Using this is faster, because the workload is distributed to internal working threads and the db operations are optimized for many objects at once.