I don't know how Python handles it, but R uses vector operations for almost every function, meaning that for large data sets (10000+), the efficiency of data manipulation in R is always O(1), and may otherwise be O(n) in Python.

I have a function in R that given a list of keywords, for each word (~100 keywords), it checks a data frame that stores a list of keywords (~5000 lists of keywords, ~10 keywords/list) to see which keywords matches the keywords in each row of the data frame.

Efficiency using straight for loops would be O(mno) and could take awhile. But in R, all those operations are vectors, and it takes less than a second.