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Accessing primes[i] is done in constant time, O(1). What that means is that the time needed to read primes[i] does not increase as the primes becomes bigger and that it does not increase when i becomes bigger. In layman's terms: it's damn fast!

Then again, accessing a local variable p is still faster than accessing primes[i], because the latter has to look up and call the __getitem__ implementation of the primes object. Therefore caching a value in a local variable instead of looking up a list twice is marginally faster.

On the other hand, caring about marginal speed improvements is meaningless compared to reducing algorithm complexity. For the problem of finding prime numbers, you should focus on finding a smart algorithm rather than on improving built-in-list access times.