INSERT with CLOB column resulting in high db file sequential read events

Hi,

One of our Performance Test DBs has started to show an unusual (IMO) behaviour under a certain circumstance, namely:

A single record INSERT statement is taking 30-45 seconds to complete, due to a high volume of db file sequential read Wait events.

This isn't consistently happening for all INSERTs - however, having spent some time investigating, I can consistently replicate it: The table contains 2 CLOB columns and:

1. If the combined length of the CLOBs is less than a specific value, the scenario happens
2. If the combined length of the CLOBs is greater than said values, it does not - the INSERT runs in <100 ms.