What is CompilationCache.noindex in DerivedData?
Quick answer
~/Library/Developer/Xcode/DerivedData/CompilationCache.noindex stores results for Xcode's compilation caching, the build setting COMPILATION_CACHE_ENABLE_CACHING (it appears in Xcode's Swift and Clang build settings). It only grows when that setting is on. It's a cache: safe to delete; builds just lose the cached results.
What it is
Compilation caching lets Xcode reuse the result of compiling a file when nothing about it changed. The results go here. On our test Mac, where the setting wasn't turned on, the folder was only 28 KB.
Check the setting
In your target's build settings, search for compilation caching, or look for COMPILATION_CACHE_ENABLE_CACHING in your .xcconfig files.
Can I delete it?
Yes, with Xcode closed:
rm -rf ~/Library/Developer/Xcode/DerivedData/CompilationCache.noindexQuestions
Why is my CompilationCache.noindex empty?
It only fills up when compilation caching is enabled for a project you build.
Does deleting it slow down builds?
Only until the cache is filled again, and only if the setting is on.