Upgrading to Xcode 13.2.1 seems to have fixed long launch times for our macOS app ! I had been waiting for a new update of BigSur to finally fix this issue. However, I have now updated to macOS 11.6.4, and Xcode 12.4 and the extremely slow startup our our large macOS app attached to the debugger was still 8 to 10 times slower than before upgrading to 11.3. Yesterday I decided to try Xcode 13.2.1. Our team has not officially switched to this compiler yet for but I can build and debug with a newer Xcode even though our CI computers are still using an older version of Xcode. I was initially a little disappointed because the full build time for the our very large macOS app was slightly slower than before upgrading to BugSur 11.3 and the older Xcode. However, I can forgive that as a minor inconvenience because now launch and begin debugging almost as fast as I was before upgrading to 11.3!

The slow down in Debugger launch times can be even worse for large macOS applications. It now takes ten times longer to startup our application after upgrading to BigSur 11.3 (and 11.3.1). Previously, the splash screen was visible almost immediately; however, it could take another minute or so to load the hundreds of dylibs, frameworks and plugin bundles to complete the application's initialization. With this slower dyld in macOS 11.3 (and 11.3.1), the application takes over a minute before the splash screen is displayed and another 9 or 10 minutes to finish loading all the components. (This is using an iMacPro 2017, 2.5 GHz 14-Core Intel Xeon with 64GB.) The bundles in particular are now loading very slowly. Every time I break into the debugger the top of the stack looks like this: Main Thread (1) Queue : com.apple.main-thread (serial) #0 0x00000001183fb532 in _dyld_debugger_notification () #1 0x00000001183fa6bb in gdb_image_notifier(dyld_image_mode, unsigned int, dyld_image_info const*) () #2 0x00000001183fa3af in notifyGDB(dyld_image_states, unsigned int, dyld_image_info const*) () #3 0x00000001183ecf51 in dyld::notifyBatchPartial(dyld_image_states, bool, char const* (*)(dyld_image_states, unsigned int, dyld_image_info const*), bool, bool) () #4 0x00000001183fc5a9 in ImageLoader::link(ImageLoader::LinkContext const&, bool, bool, bool, ImageLoader::RPathChain const&, char const*) () #5 0x00000001183ed519 in dyld::link(ImageLoader*, bool, bool, ImageLoader::RPathChain const&, unsigned int) () #6 0x00000001183f8d63 in dlopen_internal () #7 0x00007fff20626c94 in dlopen_internal(char const*, int, void*) () #8 0x00007fff2061507e in dlopen () #9 0x00007fff20728534 in _CFBundleDlfcnLoadBundle () #10 0x00007fff207a18da in _CFBundleLoadExecutableAndReturnError () #11 0x00007fff2076932a in CFBundleGetFunctionPointerForName () At the moment it is not cost effective to build and test our application on macOS 11.3 or 11.3.1. Hopefully Apple will have a fix this Debug performance issue asap.