Looking for even more performance; realistic?

[JMTC]

While I spend considerable time on implementing every possible speed optimisation I can think of, the results disappoint me.

Before implementing all optimisations, the load time was 0.665 seconds and start render time 1.089s:
http://www.webpagetest.org/result/170414...1/details/

With all optimisations, performance increased marginally to a load time of 0.606s and a start render time of 0.892s:
http://www.webpagetest.org/result/170421...5/details/

Here's the visual comparison between both tests:
http://www.webpagetest.org/video/compare...414_XR_H2J

I have two questions that I'd like to get your opinion on:

(A) Is it realistic to bring down the start render time further? I was hoping of hitting 0.6-0.7 seconds.

(B) When it's realistic, what other optimisations can I pursue? I can honestly say that I implemented everything I know, so I'm not sure how to proceed here.

Thanks very much for your input!