Slowdown will turn fairly quickly into a major speedup if you're trying to do operations on multiple files.

You can also configure zsh to show you all the autocomplete options (files/directories, flags, command-specific arguments like git tags, etc.) and tab/arrow key through them visually: http://i47.tinypic.com/2qtwxnn.gif

(The gif is a little slow, but you can obviously move through them a fast as you can hit the keys)

In my experience, the slowdown is usually not remembering the command, but remembering, say, whether you want -f, -F, --force, --hard or some other option for this particular command.

Even common commands have this problem — for example, I don't copy files all that much, so when I do, I frequently forget that cp takes -R instead of -r.