It's the difference between having dual-precision operations implemented in hardware and having to do a dual-precision operation as four single-precision operations.

For example, consider multiplying "16" by "43". If you had a native dual-digit multiplier, you would do it in one operation. If you only had a single-digit multiplier, you'd need to multiply 1 by 4, 1 by 3, 6 by 4, and 6 by 3, to do it. Like this:

```
16
X 43
----
18 (6*3)
3 (3*1)
24 (4*6)
4 (4*1)
-----
688
```

Notice that you needed to do *4* single-digit multiplications to do one two-digit multiplication, so it takes four times as long. So, if you don't have a dual-precision multiplier but only have a single-one, it takes you four times as long to do a double-precision multiplication.