Insert (write) performance
Table structure-wise, you are not doing anything wrong that would slow down inserts:


You (presumably) insert in PRIMARY KEY order via the auto_increment value.
There are few secondary indexes on the table.  Both posted_by and posted_to are non-unique, which means InnoDB can use the insert buffer.


I would suggest looking at either configuration or hardware.  I would agree that 200ms is a lot, so it looks like there has to be some contention somewhere.