We run 5 site on the same Magento platform (2.1.6) and I have recently been trying to optimize the speed of our site using the parameters outlined by Google Page Speed Insights. Having struggled to really eliminate any of the things google dislikes I wanted to see how other Magento 2 site performed. They ALL seem to perform poorly on the PSI analysis. This includes sites developed by Magento Gold developer partners. Any comments would be most appreciated....
So much for Magento Gold Developer partners. I think they just don't care about PSI. Some don't care about performance benchmarks at all.
Without giving out names I was approached by a client whose site was migrated to M2 from M1 by some Magento partner. They complained about poor TTFB (time to first byte). Turned out they had an extension that adds 300ms to TTFB for any page, cached or uncached.
A fresh copy of Magento 2 with Luma theme and with FPC on scores above 80% which is pretty good. It is when they create a custom theme and add 3rd-party extensions things start to look bad.
Bonus: Here is an article I wrote that would help your speed up Magento 2 - https://www.goivvy.com/blog/speed-up-magento .