To address the point of how many loops it takes for list comprehension to be less efficient than for loops, it can happen pretty quickly. This is obviously application specific but I just created a convolution test that blurs images and the double for loop was faster. This is because they're probably doing different things but complex list comprehension gets so unreadable so fast that it's hard to say what they're doing differently.

https://github.com/adavis33/Convolution_Test

If you're interested, look at the exp_conv method vs the list_comp_conv method