Come posso velocizzare questa query SELECT CONCAT/GROUP BY?

Crea un indice composito su (state, city) e riscrivi la tua domanda in questo modo:

SELECT  CONCAT_WS(', ', city, state) AS location, AVG(latitude), AVG(longitude) 
FROM    places
WHERE   state='NY'
        AND city='New York'
GROUP BY
        state, city

Nota che per questa stessa query puoi omettere GROUP BY clausola:

SELECT  'New York, NY' AS location, AVG(latitude), AVG(longitude) 
FROM    places
WHERE   state='NY'
        AND city='New York'

Tuttavia, questa query ne avrà ancora bisogno:

SELECT  CONCAT_WS(', ', city, state) AS location, AVG(latitude), AVG(longitude) 
FROM    places
WHERE   state='NY'
GROUP BY
        state, city