One note: the performance of `Quantity` has improved by leaps and bounds since its initial implementation, but it would still be dominant for ccm89, od94 and gcc09 if used this way:
```python
In [24]: wave = np.random.rand(100) * u.AA

In [25]: %timeit wave.to(u.AA, equivalencies=u.spectral())  # would do this to input wave
10000 loops, best of 3: 73.6 µs per loop
```
(compare to 30-40 microseconds for ccm89 for small arrays)

If spectral equivalencies are not allowed, the conversion time is subdominant:
```python
In [26]: %timeit wave.to(u.AA)
100000 loops, best of 3: 19.3 µs per loop
```