Enormous CPU times using large DECIMAL_PLACES with hex numbers

Seeing some enormous CPU times when using high `DECIMAL_PLCAES` config with hex numbers. The following code takes ~30s to finish on a high end computer.

Removing the `DECIMAL_PLCAES` option (or making it smaller), maker things better.

```js
import { BigNumber } from "bignumber.js";

BigNumber.config({
  DECIMAL_PLACES: 1e9
});

BigNumber("0x9a864287d")
```
