Anyway, instead of running /usr/bin/login, I just use /bin/zsh as my Terminal.app startup command which is much faster.
However, every time I access the file system, even some tab for autocompletion, it takes a few seconds. Even cd'ing into some directory can sometimes take a few seconds. Sometimes ~= it's more than 1h ago that I accessed that dir or so.
Edit: Maybe it's Time Machine or Spotlight or so which destroys the effectiveness of the VFS cache?