Conversion from RGB to CIELAB with a complex formula
The main thing that would speed it up is to not constantly calculate constant values. For example, you can replace:

pow ((6.0 / 29.0), 3.0)


with 

0.00885645167904


Preferably you'll give it a name like:

let threshold = 0.00885645167904


Likewise for the other call to pow(). pow() is a very expensive function to compute, so removing it can be a big win.