Improve BLOB Writing Performance in Oracle 11g
In Oracle, LOB (including BLOB) is stored as:


in-the-table LOB - if the LOB is smaller than 3900 bytes it can be stored inside the table row; by default this is enabled, unless you specify DISABLE  STORAGE IN ROW
normal LOB - stored in a separate segment, outside of table, you may even put it in another tablespace; for these:

a minimum of CHUNK bytes are allocated and entirely redo-logged (even if LOB has only 1 byte)
there is an internal intermediate index behind a LOB column, which gets contentious on updates and may practically serialize them
access is multi-level and thus relatively slower
NOCACHE is the default
with CACHE option, the CACHE_SIZE_THRESHOLD is not taken into account, so a large LOB can waste your cache



The Metalink note ID 66431.1 describes this and may be of interest to you, if you have access there.