⚡️ Why Is This Loop So Slow?

Imagine you're checking whether thousands of usernames exist.

for username in usernames:
    if username in banned_users:
        ...

If banned_users is a list, Python checks one element at a time.

Alice?

Bob?

Charlie?

David?

...

For every lookup.

Now imagine banned_users is a set. Python doesn't search one by one. It uses a hash table to jump directly to where the value should be.

That's why changing this:

banned_users = [...]

into this:

banned_users = {...}

can dramatically speed up membership checks without changing the rest of your code.