ReSharper is one of the most powerful tools in the .NET ecosystem. For years, developers have loved it for its accurate code analysis, extensive refactoring support, intelligent navigation, and deep integration with the .NET language stack. But ReSharper also earned a less enviable reputation of being slow and resource-hungry, especially on large solutions. One year ago, I joined JetBrains to help with the ongoing effort to fix ReSharper. In this talk, we will explore what it means to optimize a 20 years old codebase with millions of lines of code. The approach we took, the priorities and compromises, the custom tooling we developed to support the effort. We will see what made ReSharper so slow, the solutions we implemented, and where we stand today. We will look at concrete examples, from simple inefficiencies that would be found in most codebases, to deep technical issues that arise from running inside Visual Studio.