[Python] What is the most efficient way of finding the index of an item from a list of lists...

I have the following list:

['F1', [10, 'r'], 'F2', 'F5', [15, 'w'] 'F3', [14, 'r'], 'F4']

I want the index of the list [15, 'w'] using the value 15.
What would be the most efficient way of obtaining the index value? (answer should be 4)

I tried using lambda x: x[0] inside the python .index() function but could not figure out anything of value. I do not want to use loops and if statements. Is there a way to efficiently carry out this task using python's in-built functionalities?

The first value of every list will be unique for all lists inside the main list.

Edit:
By saying "I do not want to use loops" I meant nested loops and if-statements which increase the complexity of the solution, since I will be working with very large lists in many places of the code. So one loop would be fine as long as the complexity isn't greatly increased.

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