"Disabling as many extensions as possible increases the speed of your scripts: true or false" True, but not massively. Each extension has to do some work each time a request comes in, but it's very minimal.
"Adding indexes to every field in your table maximises the chance of top performance: true or false" False: overindexing is almost as bad as not indexing at all!
"What is the difference between a DELAYED query and a LOW_PRIORITY query" DELAYED queries are not executed immediately, but are queued up until a sufficient number of queries is ready or a certain amount of time has passed, then are executed at normal priority. LOW_PRIORITY queries wait until no more reads are waiting.
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