There have been plenty of major performance improvements for Ruby but most of them require changes that just aren't feasible for a large codebase. For example, there are alternative Ruby interpreters that are far faster than the MRI but they're not usually a drop in replacement because they don't have full Gem or really general ecosystem compatibility.

Also, some performance improvements like JIT just don't matter that much for Rails because the primary problem isn't single thread performance it's the lack of cheap parallelism.