A few percent higher performance is not something that will buy you customers. Especially that after you finish your rewrite, the original will have more featur

A few percent higher performance is not something that will buy you customers. Especially that after you finish your rewrite, the original will have more features, better stability (already used and proven in production) and... very likely better performance. The biggest impact on performance has architecture an quality of algorithms, not the low-level aspects. People still like Java Minecraft better than thousands of its C++ clones.