2.X vs 1.6.3 : what's the difference?

[fpglt]

1.6.3 is definitely a standard for me. It works out of the box, has much acceptable loading time and never causes Vulkan device loss events. It stutters from time to time, mostly as I approach destination airport but nothing unbearable. All that at ZL17, whether I use a Simbrief plan or not. And I don't change the settings with respect to the 2.x I used, but it works great.
So after testing each 2.x versions with which, despite tinkering with settings, I end up with missing tiles and Vulkan errors, I keep reinstalling 1.6.3, and restart with a clean cache.

I know there has a been a code rewrite between the two version series but what can cause a performance hit like in the 2.x versions?