It's not about the size of the registry. It's not about the speed of the PC. Performance can mean different things.
The issue is this:
Orphaned Registry Entries can cause issues when you reinstall or upgrade an application.
I'm having issues with Visual Studio 2015 right now. There are over 1,000 registry entries left after uninstallation and reinstalling it leaves the IDE unusable. It just gives tons of errors about not being able to find files (that it is likely being told
the location of in those registry entries), among other things. The only way to get it to work is to "Repair" the installation, except that's completely broken because if you only install Visual Studio and Visual C++ w/MFC it will then enable ASP.NET on your
machine. It will install all manner of Windows SDKs (Windows 10 SDK and Tools, SQL Server Database Tools, etc.).
If you develop on a machine with a 128GB SSD (which is otherwise fine, since you used the custom install option), suddenly you see this product using over 1/3rd of your usable hard drive space - in some cases, you go from having 10GB of free space to something
like... Well, you don't have enough space to "repair" the installation because of the extra **** the installer is trying to install.
The only way to fix this, is to use a deep registry cleaner to completely remove the Visual Studio 14 Registry Entries after uninstalling it (Manually, because Microsoft can't be convinced to write a decent uninstaller).
This is a platform issue. When someone uninstalls an application, the application should be completely uninstalled. Orphaned Entries in the Registry do cause issues. I've also seem PCs that would lock up immediately whenever LibreOffice was launched because
of orphaned registry entries. Then there are issues with Driver Settings after Uninstall/Reinstall or Upgrades, among other things.
There are so many issues that are registry-oriented. The "Repair" option for most installations is used likely due to issues caused in the registry more than actual issues with files in the Program Files directory, these days.
The registry system is a hot mess. They should have replaced it with something different a long time ago, IMO.
There's a big difference between managing Files/Settings/App Installations on OS X than on Windows, for example. Complete night and day.
I have literally never had an application break simply because I uninstalled it and reinstalled it at a later date on OS X. This happens all the time on Windows (2000, XP, Vista, 7, 8.x, and 10... It's been a perpetual issue ever since the applications
moved full bore into throwing things into the registry instead of using their own INI files, which one could easily hunt down and delete - disregarding issues with library registrations, etc.).
A lot of the information in this thread are being repeated by people who really don't have much experience dealing with issues derived from those situations. They are merely repeating what they have read, and what Microsoft wants them to believe because
the Registry is a hot mess and they are too lazy to move to a superior (and/or simpler) model; and seem incapable of really developing their own suitable utility to fix it. They just tell you to System Restore and hope the issues are going to go away.
I haven't heard of anyone who's had to reinstall OS X because of plist issues. I've had to reimage more than enough Windows PCs because of Registry issues making them run like ****. Not performance wise, but in terms of blocking/breaking program installations/functionality,
random error messages at boot and shutdown, etc.