MultipartReaderStream currently uses a Boyer-Moore substring search implementation as part of finding the next boundary:

We should check to see whether this is still more beneficial than just using the vectorized:
https://github.com/dotnet/runtime/blob/7b91fd42a64732681472afc8d5a52c5bc5eb0c8a/src/libraries/System.Private.CoreLib/src/System/MemoryExtensions.cs#L1694

Regex used to use Boyer-Moore as well, and in .NET 7 deleted its use of Boyer-Moore entirely, instead just using IndexOf, for significant gains in most cases. See https://devblogs.microsoft.com/dotnet/regular-expression-improvements-in-dotnet-7/#leading-vectorization for details.