stack exec is slow

### Steps to reproduce

Outside of a stack project:
```
time stack exec echo 'hello'
Run from outside a project, using implicit global project config
Using resolver: lts-7.2 from implicit global project's config file: ~/.stack/global-project/stack.yaml
hello

real 0m0.385s
user 0m0.459s
sys 0m0.070s
```
Inside a small stack project with the only dep being base
```
time stack exec echo 'hello'
hello

real 0m3.643s
user 0m3.620s
sys 0m0.135s
```
Any ideas to make this faster? Dev tools like hdevtools using stack to do their job consume very most of their time waiting for stack, which is unfortunate.
### Expected
Faster startup time of stack
### Stack version

```
stack --version
Version 1.2.0 x86_64
```

### Method of installation

* [Nixpkgs](http://nixos.org/nixpkgs/)