Totally depends on the use case I suppose, we found that in our environment, we perform _a lot_ more SELECT's than we do UPDATE/DELETE/INSERT's.

And with some badly optimized SELECT's, the time MySQL had to spend on sorting results/reading from disk in an inefficient way made all our _write_ queries suffer.

By optimizing our SELECTs first, we freed up some CPU bandwidth (it seems?) that can be spent doing all the other work.