Every time I've investigated an issue like this, it was something outside of explorer. Most of the time some kind of plugin that I didn't want (PDF readers, Tortoise SVN/Git, document preview handlers, you name it), mapped network drives not responding, or file system corruption that went by seemingly unnoticed.
At some point I started Autoruns and just disabled every DLL that didn't come from Microsoft and that I didn't strictly need. It sped up Windows immensely, and I went back enabling maybe one or two of those DLLs at a later point because it broke functionality.
I could've saved weeks of my life if Microsoft had just added a little popup that says "7zip.dll is adding 24% extra time to opening right click menus, disable it Y/N" every time a badly programmed plugin slowed down my shell.
At some point I started Autoruns and just disabled every DLL that didn't come from Microsoft and that I didn't strictly need. It sped up Windows immensely, and I went back enabling maybe one or two of those DLLs at a later point because it broke functionality.
I could've saved weeks of my life if Microsoft had just added a little popup that says "7zip.dll is adding 24% extra time to opening right click menus, disable it Y/N" every time a badly programmed plugin slowed down my shell.