Slow startup time when using plugins

What is the current behavior?
-----------------------------
Currently if you install some plugins the CLI slows down considerably, for example the heroku-pg-extras plugin:

### Before

```bash
heroku ps -a myapp 0.49s user 0.27s system 33% cpu 2.316 total
```

### After

```bash
$ heroku plugins:install heroku-pg-extras
heroku ps -a myapp 1.33s user 1.32s system 41% cpu 6.377 total
```

What is the expected behavior?
------------------------------
I would expect some commands like `ps` would not load the entire plugin suite, saving time.