I'll try to answer these in order.

• It depends. How many is "lots"? How slow is "slow"?
• It depends. What do you mean by "suffer"? Do you need those millions of rows? How are your indexes? What does the query plan look like?
• It depends. How does the query plan look? Are you joining the tables on primary keys, or at least indexed columns?

Now I have one question for you. Have you found your application to be too slow, profiled it, and noticed that after tuning MySQL appropriately and normalizing your tables, it's still too slow?

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