display long durations in seconds

The duration display for particularly long durations seemed to be incorrect. I switched it to show time in seconds out to two decimal places which I think was the original intent.

Previously:

```
> "%.2fms" % 1500
=> "1500.00ms"
```

Now:

```
> "%.2fs" % (1500 / 1e3)
=> "1.50s"
```