Locking because of triggers mysql
In one situation, I found it possible to ingest data faster into InnoDB than into MEMORY.  Perhaps you should simply get rid of the MEMORY table and the TRIGGER.

I discuss that an many other tidbits in my high-speed ingestion blog.

Do you have multiple inserters?  Do you insert multiple rows in a single INSERT?  Are you pulling the data out of the MEMORY table?  How often, how, when, etc?  Is Replication involved?  Is "normalization" happening?  Summarization?