Building a package is slow

I recently discovered that building packages is surprisingly more costly than building modules. See https://github.com/commercialhaskell/stack/issues/2831 for a benchmark and results. The takeaway is this:


I chased that behavior down to `cabal configure` and `cabal build`, so the problem isn't really with Stack. That made me wonder about how long it takes to build a package with Cabal, so I wrote [a benchmark](https://gist.github.com/tfausak/e431b194eb4374184281ae1769004638) for that. Here are the raw results:

```
benchmarking cabal sandbox init; cabal sandbox delete
time 152.6 ms (146.5 ms .. 156.6 ms)
0.998 R² (0.992 R² .. 1.000 R²)
mean 155.7 ms (152.5 ms .. 159.4 ms)
std dev 4.734 ms (2.695 ms .. 6.772 ms)
variance introduced by outliers: 12% (moderately inflated)

benchmarking cabal sandbox init; cabal build; cabal sandbox delete
time 1.414 s (1.362 s .. NaN s)
1.000 R² (0.999 R² .. 1.000 R²)
mean 1.433 s (1.422 s .. 1.443 s)
std dev 17.97 ms (271.9 as .. 18.36 ms)
variance introduced by outliers: 19% (moderately inflated)

benchmarking cabal build
time 495.2 ms (448.5 ms .. 532.5 ms)
0.999 R² (0.996 R² .. 1.000 R²)
mean 502.3 ms (495.4 ms .. 507.5 ms)
std dev 8.000 ms (0.0 s .. 9.039 ms)
variance introduced by outliers: 19% (moderately inflated)
```

What I take away from that is this: Building an empty package with a cold cache takes 1.4 seconds on my machine. A warm cache brings that down to 0.5 seconds. Both of those seem high, but especially the former. Are these results expected?