Found by working on 'DJ as a shared-repo' - IDPAAS-65. IDM is using REST2LDAP which is configured with MVCC enabled. Result is that etag is systematically computed.
The EntityTagVirtualAttributeProvider is computing the etag value of an entry using Adler32 or CRC32.
The problem identified is that the computing is performed byte by byte, which is quite costly given that the update(int b) method is native.