You might also benefit from increasing FREELISTS from the default.
Start with FREELISTS at 2 and see whether your concurrent inserts are still locking.
This is a locking contention issue, so getting familar with the locking mechanism and reporting lockers/blockers will help determine if your settings for INITRANS and FREELISTS are too low.
Performance... Push the envelope!
Yes, freelists might help but mostly in OPS, i.e., when several instances access the same table. And consider then not only FREELISTS but also FREELIST GROUPS.
About INITRANS, if you are familiar with the ORA-08177 error, then the INITRANS problem is not new to you. It occurs when the transaction has to reread the index block and try to generate a consistent read version of that index block.
Some claim that high INITRANS would help, I have seen from practice that .. not always. Oracle says it is some sort of a bug.
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