At work, I'm the maintainer of a 30k SLOC VB.NET codebase. Originally windows-only, when I took over I ran it under mono on Linux and OSX. The day that MS released dotnetcore, I jumped to that. Overall the experience has been really good.
There are lots of complexities, but in short... .NET 5 gave it a 4% speed boost across the board, going up to 20% [!] in some cases. Mainly that was ascribable to the intersection of GC improvements and kinda-degenerate cases in the model.
For this particular codebase, very little in that article will be that significant [the VB.NET code uses a pretty limited set of stuff in the core libraries]. But their trajectory, and a million tiny improvements, really gives me a lot of optimism.
What I originally considered a pretty cynical stab at Oracle when they released .NET's source has really turned into a wonderful, performant, developer-friendly, and pleasantly-cross-platform ecosystem.