Major upgrades that are fully backward compatible and therefore leave some significant performance and safety bugs untouched. What they mean by "slow evolution"

Major upgrades that are fully backward compatible and therefore leave some significant performance and safety bugs untouched. What they mean by "slow evolution" is the increasing rigidity of core data structures and resistance to introducing replacements caused by the backwards compatibility promise. Adding new features is easier than changing existing ones. This is both a strength and a weakness of C++. It's a very old language and it has been keeping that backwards compatibility promise for a very long time. The result is that certain portions of the language are effectively unchangeable as a result. This is the key thing that Rust's editions are trying avoid.