@BruceForstall has done some great job optimizing BitArray in dotnet/corefx#39173 . The PR was closed because Bruce has currently no time to finish it.

A contributor who would like to work on this issue should:

• pick Bruce's work from Speed up BitArray corefx#39173
• finish the todo mentioned in this comment: Speed up BitArray corefx#39173 (comment)
• run these micro benchmarks from the performance repo and provide updated results (before & after)

The issue should be a great excercie for somebody who wants to learn more about vectorizing code using the new .NET Core 3.0 CPU Intrinsics API