What are possible causes for consistently volatile INSERT performance in MySQL tables?
The simple answer is: it is not possible to guess what is the source of contention.

The more detailed: MySQL 5.6+ can instrument a break-down of query execution time, so that you can see if performance is stalled waiting on IO, locks, etc.

The diagnostic feature is called performance_schema.  The easiest way to start using it, is to download MySQL Workbench 6.1 and chose "Performance Reports" (under Performance).