Having tested mono 6 both for stability and performance well over a week, I have found mono 6 good to go for both building and running OpenSim on macOS.
From the mono 6 Release Notes there are some new functionality for LLVM (which is the compiler used by Apple) that might add better performance, so that has to be looked into. Mono on macOS is largely using LLVM and not the default mono JIT.
For building OpenSim on macOS use
It is worth noticing that mono 6.0.0.319 is also the version of mono that is installed on your system if you install Microsoft Visual Studio for macOS, meaning you can build and debug OpenSim in VS.
You can also place the following lines in your .profile or .bash_profile to improve mono performance somewhat
From the mono 6 Release Notes there are some new functionality for LLVM (which is the compiler used by Apple) that might add better performance, so that has to be looked into. Mono on macOS is largely using LLVM and not the default mono JIT.
For building OpenSim on macOS use
msbuild /p:Configuration=Release
for best performance unless you specifically want to debug the build.It is worth noticing that mono 6.0.0.319 is also the version of mono that is installed on your system if you install Microsoft Visual Studio for macOS, meaning you can build and debug OpenSim in VS.
You can also place the following lines in your .profile or .bash_profile to improve mono performance somewhat
export MONO_GC_PARAMS="nursery-size=32m"
export MONO_THREADS_PER_CPU=8