regarding transaction commit in mysql?
Beyond some point, waiting too long to COMMIT slows things down.  Think of it as having to save more and more "undo" stuff.

innodb_autoinc_lock_mode only applies to AUTO_INCREMENT.

It seems that your real question is about high speed ingestion.  Are you INSERTing one row at a time?  You can get 10x speedup if you insert 100 at a time.  That is, a single INSERT with 100 rows.  Are you inserting from multiple connections?  Etc.  Here's one discussion on high speed ingestion.