You shouldn’t worry about performance unless your code is in a tight inner loop, and is actually a performance problem. Instead, use code that best expresses your intent. Some people like lambdas, some like itemgetter. Sometimes it’s just a matter of taste.
itemgetter
is more powerful, for example, if you need to get a number of elements at once. For example:
operator.itemgetter(1,3,5)
is the same as:
lambda s: (s[1], s[3], s[5])