Room to optimize fast path for Haskell shell scripts run via stack?

I absolutely love being able to make self contained shell scripts like this:

``` Haskell
#!/usr/bin/env stack
-- stack --verbosity silent --resolver lts-3.8 --install-ghc runghc --package turtle --package filemanip --package optparse-applicative
module Main where
import Turtle
main :: IO ()
main = putStrLn "hello world"
```

But the above hello world takes up to 2 seconds to run on my laptop, even after the first run where the installs occur. Could we get a description of what the fundamental work to do in this scenario is? Which parts of it _must_ be slow, and which parts may have room for improvement?

For example, I would hope the fast path would basically say:
- is snapshot `lts-3.8` there already?
- are the requested packages installed already? (3 checks)
- go!

And it sounds like those should be `O(1)` operations, checking only on a few packages.

I'm guessing the current algorithm is more conservative, running some kind of `O(N)` sanity check over, e.g. all the transitive dependencies? If it's something like that, should we work on a fast or trusted mode where only the constant time checks are done?