We got this blurb at the end of the Patch Preview:
This may or may not just be referencing the loot label optimization that we know of or there could be further optimizations coming in this patch that we don’t yet know about.
The big optimization will likely be the spell effects auto-pooling system (mentioned in the “Possible Inclusions” section of the OP), but we’re unsure if it’ll be included in this patch or if it’ll be coming in a future patch.
Mike talks about this optimization in the February 26th Q&A, the March 19th Q&A, the March 26th Q&A, and it’s actually brought up a lot during the April 28th Q&A/VFX stream. The February 26th Q&A probably has the most detailed response about it (see the “When are you doing some big performance pass?” question).