The question still remains: what are they optimizing for if we already have sub-millisecond response times?
The only evidence you offer is anecdotal. I'm having a hard time imagining perceptible differences in sub-millisecond response times so either you or the study are in error.
And just to reiterate other comments: yes, Safari is probably able to block ads just fine now, because they modeled the engine according to capabilities needed right now. However, it comes at a huge cost to innovation and creates yet another barrier of entry guarded by a large corporation.
Taking all of that in account, it seems unnecessary. The looming threat of ulterior motives from Google remains ever-present.