A number of the CultureInfo benchmarks show a major performance loss on Mono compared to .NET.

We see a time of 9299 ns on Mono where .NET Core has a time of 19 ns.

The below benchmark exhibits that.
https://gist.github.com/alexanderkyte/0fb4a79cc3c5edbf4af6e7714ffaebe2

It doesn't look like this is a codegen issue as much as a bit of sub-optimal managed code.