This question already has an answer here:
There is no XSLT 2.0 processor offered by Microsoft.
An XSLT 2.0 processor that claims to be written especially to be used in .NET is XQSharp.
Another very popular XSLT 2.0 processor is Saxon 9.x, which, although written in Java, has a special .NET version.
I disagree that XQSharp (or for that matter Saxon) is slow. While neither currently compile to MSIL, both have good optimizers. I know from first-hand experience thatlarge grouping tasks can be faster with XQSharp than with XslCompiledTransform.