Seriously, they built a custom-built physics engine from scratch that seems to perform much better than the current ones out there (PhysX, Bullet, etc.). This alone is an achievement of years and years of R&D… (For examples of the inner details see https://youtu.be/P-WP1yMOkc4 or some of their technical blog posts)

And the stuff doesn’t end there. Nowadays they also have their own Lua-derivative language forked from Lua 5.1 (https://luau-lang.org/) with many improvements like static typing, performance, and sandboxing. And their renderer seems to work smoothly on low-end computers too, they've probably done lots of optimizations over the years.