As someone who deals with this, Framework -> Core on Windows is small % performance improvement. Framework Windows -> Core on Linux is huge. Most of it coming from not Windows.

Yes, there is other nice language features but obviously 15 years of Framework code base has probably put up guard rails around those sharp edges.

My point still stands, I can't imagine most companies green lighting .Net Framework -> Core conversion if they can't switch to Linux. If you are stuck on Windows, you have probably developed all the tooling to deal with Windows so it's all sunk costs.