The 'GS_THREADSAFE' flag is for when you build Ghostscript, it is not a flag for ImageMagick. The binary release published by Ghostscript is not build with that flag. I could create a custom binary that has this flag but I don't really have the time to do that now.
The conversion should become a bit faster when you use the next release. I hope I can publish it this week.
The conversion should become a bit faster when you use the next release. I hope I can publish it this week.