DB2 should be many times faster than DB1. If it isn't, you're doing something wrong. For one thing, I see you put in placeholders for the SQL but don't seem to be passing it any values. Also, the SQL is very different between these. For DB1 and DB2 you are supplying values for response but on the others you just join on respondent.

I strongly recommend that you learn how to use the EXPLAIN command in MySQL to determine if your indexes are correct. A correctly indexed table is the best way to approach this problem.

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