We have an open source project LEAN (github.com/QuantConnect/Lean). We've seen a 90% slow down in our basic benchmarks since upgrading to Mono v4.2.1
e.g. Normal Speed : v18.104.22.168 - 23k/points/sec
New Speed : v4.2.1 - 3k/points/sec
This is very repeatable. We've tested on different hardware, to confirm.
I have no proof of this but believe its from the new Threadpool as our application is intensely dependent on the threads.
To reproduce download the open source project and build using xbuild or visual studio, execute the project and read the speed output. Repeat using latest version of Mono.
Did you try running with the log profiler, and analyse the output with mprof-report (http://www.mono-project.com/docs/debug+profile/profile/profiler/) ?
If you do, please copy/paste the output for both runs, so we can compare what are the differences.
Thank you very much for your report,