Microsoft will begin to ship an in-house custom built Linux kernel starting with the Windows 10 Insider builds this summer. This kernel is to become the backbone for the new Windows Subsystem for Linux 2.0 or WSL2. The source code for the kernel will come from Kernel.org, Microsoft has stated that they will apply custom patches that reduce the memory footprint of the kernel and provide hardware compatibility. The new Linux kernel has improved the performance of WSL, with unpacking archives up to 20x faster and tools such as git, git, and cmake being 2-5x faster.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-to-ship-a-true-linux-kernel-with-windows-10-wsl/