it's a matter of using weight wisely.
when playing the piano, we put pressure on the keys in order to make it sound. we may use the weight of the fingers, hand, forearm, or whole arm, or even body (big chords) depending on how much wieght you want to put. basically, the more part of the body you incorporate, the more pressure you make on the keys, thus producing more sound. that's why a lot of teachers tell us to firm up the first joints of our fingers because these joints have to be strong in order to "carry" the weight you put into it and not collapse. (firming up these joints and relaxing at the same time is difficult to combine)
but pressure from natural dropping of weight is different from forceful pressure.

the latter one should produce a harsh sound and can be taxing.
GOING BACK TO THE TOPIC, if you need to do very light and fast runs, i think it's physically the easiest thing to do... but i do know and agree that it is easier said than done. but once you get the correct technique, you will remember it very well and will be easy whenever you need to use it again.
im saying that it's physically easy to do because playing very loud and very fast runs is hell a lot more difficult. that's why teachers tell us to play lightly on very fast passages because that would mean less movements, economy of movements, thus making you move faster to the next note/s.
SO, TRY THIS. this is how i work on such passages.. hope it works for you as well. considering that i know already the notes of the passage very well...
1) i practice it slow and loud, lifting my fingers just a little bit in order to feel which finger is playing now, and also to feel the slight stretch. now, playing loudly has a lot of tension...
2) ...so, i play it a little bit softer (by notch only), lessening the lifting of fingers (by notch), and now a little bit faster (by notch), but this time take away a little bit of the tension...
3) repeat step 2... repeat step 2.. repeat step 2 until you get to the right tempo. notice that it is a subtraction of volume and finger lifting, and an addition of tempo, all by only a notch everytime.
and remember that the weight your dealing with here is only the natural weight of your fingers. it's enough to make the sound you want coz it's very light.
and to top it off, i end the practice with a slow practice of that passage- this time in a conscious relaxed mode and super close-to-the-keys-touch-before-play action.

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you don't have to follow how i practise it, but try to see the logic of my practise technique... then maybe you can create your own steps.

good luck.
