I ran some tests this morning comparing the `for` loop version and the `fold` version. The source can be found [here](https://gist.github.com/zslayton/9506663).

Results:

`fold` version, compiled using `rustc`:
```
real    0m0.168s
user    0m0.164s
sys     0m0.004s

real    0m0.169s
user    0m0.164s
sys     0m0.004s

real    0m0.169s
user    0m0.160s
sys     0m0.004s
```


`for` loop version, compiled using `rustc`:
```
real    0m0.053s
user    0m0.048s
sys     0m0.000s

real    0m0.051s
user    0m0.048s
sys     0m0.000s

real    0m0.051s
user    0m0.048s
sys     0m0.000s
```

`fold` version, compiled using `rustc --opt-level 3`:
```
real    0m0.013s
user    0m0.012s
sys     0m0.000s

real    0m0.011s
user    0m0.004s
sys     0m0.004s

real    0m0.012s
user    0m0.004s
sys     0m0.004s
```

`for` loop version, compiled using `rustc --opt-level 3`:
```
real    0m0.005s
user    0m0.000s
sys     0m0.004s

real    0m0.005s
user    0m0.000s
sys     0m0.008s

real    0m0.005s
user    0m0.000s
sys     0m0.004s
```