Top 8 C# msil Projects

ILGPU

6 1,059 9.0 C#

ILGPU JIT Compiler for high-performance .Net GPU programs

Project mention: ILGPU VS ComputeSharp - a user suggested alternative | libhunt.com/r/ILGPU | 2023-10-27

AsmResolver

1 789 8.7 C#

A library for creating, reading and editing PE files and .NET modules.
WorkOS

workos.com sponsored

The modern identity platform for B2B SaaS. The APIs are flexible and easy-to-use, supporting authentication, user identity, and complex enterprise features like SSO and SCIM provisioning.
MonoMod

3 715 8.8 C#

C# modding swiss army knife, powered by cecil.
IL2C

1 389 0.0 C#

IL2C - A translator for ECMA-335 CIL/MSIL to C language.
ILPack

2 302 4.1 C#

Serialize .NET Core assemblies
InlineIL.Fody

1 231 7.7 C#

Inject arbitrary IL code at compile time.
DistIL

4 108 8.6 C#

Post-build IL optimizer and intermediate representation for .NET programs

Project mention: Classes vs. Structs in .NET. How not to teach about performance | news.ycombinator.com | 2023-11-04

How is that relevant to the vast majority of the code targeted by LINQ?
The niche scenario you have outlined is partially covered by a recent System.Numerics.Tensors package update (even though I believe it would have been best if there was a community-maintained package with comparable quality that can be decoupled from .NET release cycle and trade-off support/compat guarantees in favour of more aggressive improvements in the future).
The goal of LINQ itself is to offer optimal codepaths when it can within the constraints of the current design (naturally, you could improve it significantly if not for backwards compatibility with the previous 15 or so years of .NET codebases). The argument that it's not good because it's not the tool to do BLAS is just nonsensical.
There is, however, an IL optimizer that can further vectorize certain common patterns and rewrite LINQ calls into open-coded loops: https://github.com/dubiousconst282/DistIL

InfluxDB

www.influxdata.com sponsored

Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
UnityH4xx

1 35 2.6 C#

Hacks, tricks and wizardry for Unity to enhance performance or make impossible things possible ...

NOTE: The open source projects on this list are ordered by number of github stars. The number of mentions indicates repo mentiontions in the last 12 Months or since we started tracking (Dec 2020).