Hello, I encountered a performance bug in Pharo 6 under Windows 10 with Moose, starting from the Moose 6.1 (beta) image in Pharo Launcher. I migrated my code from Pharo 5 and it uses a PharoCommandLineHandler to load an MSE file, which sometimes has many (15K) warnings/errors. It was taking over a minute to run something that in Moose takes only a few seconds. To debug and make a minimal example, I removed the Moose file part of my CommandLineHandler and simulated the writing of errors (i/o) with a loop. I believe the I/O to the stderr is why the performance is bad. In Windows 10 Task Manager, I can see that the process is not using much CPU. As I recall, my code didn't have this performance problem in Pharo 5. The command in windows to execute my minimal sample is something like: C:\Users\me>Documents\Pharo\vms\61-x86\PharoConsole.exe -headless "C:\Users\me\Documents\Pharo\images\Moose Suite 6.1 (beta)\CommandHandler VM slow bug.image" mooseminer --msefile blah The last part (--msefile blah) is needed because I didn't want to remove the argument validation. Here's a copy of the image: https://www.dropbox.com/s/7cgt37lef2xh2ut/CommandHandler%20VM%20slow%20bug.i... A workaround is to run without -headless (or to not have so many errors). I hope this is useful to someone. For RMOD people I'm happy to demo this on my machine. Cheers, -- Christopher Fuhrman, P.Eng, PhD *Professor in the Département of Software and IT EngineeringÃTS (Ãcole de technologie supérieure)* http://profs.etsmtl.ca/cfuhrman +1 514 396 8638 *L'ÃTS est une constituante de l'Université du Québec*