Finder "Open With" very slow
Try rebuilding the Launch Services database, which populates the Open With menu. It can sometimes get corrupted, which can cause duplicate entries, slowdowns and other weirdness.

If you're comfortable with the Terminal, you can do this with a single command:

/System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support/lsregister -kill -r -domain local -domain system -domain user


Otherwise, a free GUI tool like Onyx can do it at the push of a button.