NGL would be nice if there was a clear link to the cases used both for OP as well as who you are replying to... Kinda get it in OP's case tho.

I measured the raw horsepower of the JIT engine itself, not the speed of the standard library (BCL). My results show that the Mono engine is surprisingly capable when executing pure IL code, and that much of the 'slowness' people attribute to Mono actually comes from the libraries, not the runtime itself.

In contrast, the posted article uses a very specific, non-standard, and "apple-to-oranges" benchmark. It is essentially comparing a complete game engine initialization against a minimal console app (as far as I understand), which explains the massive 3x-15x differences reported. The author is actually measuring "Unity Engine Overhead + Mono vs. Raw .NET", not actually "Mono vs. .NET" as advertized. The "15x" figure comes very likely from the specific microbenchmark (struct heavy loop) where Mono's optimizer fails, extrapolated to imply the whole runtime is that much slower.