The answer to the problem of co-contraction is here: Have a look.
https://www.pianoforum.net/smf/index.php/topic,8335.msg84684.html#msg84684(circular movements to avoid co-contraction)
A second important component of speed is simply knowing where to next place your fingers. Experiments done between accomplished pianists and non pianists showed that there were no significant differences between finger agility/speed between both groups. The reason the non-pianist could not play passages as fast was simply because they did not know the passages as well as the accomplished pianists.
A third important component of speed, is that in order to sound fast, you do not need to move fast, you have to
move smaller. A lot of the effortlessness displayed by superlative pianist is due to the fact that even at breakneck speed, they are actually moving slowly. But their movements are very small.
There are more components of speed you can read about here.
https://pianoforum.net/smf/index.php/topic,3371.msg30141.html#msg30141(zoom-zoom – all about speed playing)
https://pianoforum.net/smf/index.php/topic,3778.msg35061.html#msg35061(Speed: discussion about gradually speeding up with a metronome or using Chang’s approach)
https://pianoforum.net/smf/index.php/topic,4851.msg47341.html#msg47341(Practising for speed – Example: Beethoven Op. 49 no. 2)
https://pianoforum.net/smf/index.php/topic,5756.msg56146.html#msg56146(Talent – Attention to detail – example of scale in clusters and how the overlooked aim is to prepare fingers.)
https://pianoforum.net/smf/index.php/topic,2920.msg25568.html#msg25568(how to play superfast scales)
https://pianoforum.net/smf/index.php/topic,2313.msg19807.html#msg19807(Speed of scales – the important factors in speed playing and an alternative fingering for scales).
https://pianoforum.net/smf/index.php/topic,2076.msg17157.html#msg17157(Speed – the3 most important factors)
https://pianoforum.net/smf/index.php/topic,2079.msg17335.html#msg17335(Hand tension – not using fingers to play)
https://pianoforum.net/smf/index.php/topic,1808.msg23879.html#msg23879(Ultra fast arpeggios – discussion on slow practice x slow motion practice – good post by Herve – Abby whiteside is also mentioned)
https://pianoforum.net/smf/index.php/topic,4282.msg39831.html#msg39831(How to increase speed: slowly using metronome x fast with chord attack – analogy against slow practice using juggling and skiping rope as examples)
https://pianoforum.net/smf/index.php/topic,4144.msg40259.html#msg40259(improving speed of LH – moving the whole LH not only fingers)
Go to xvimbi’s profile and read all of his posts.
By the way, building strength decreases speed. Just look at runners and compare them with weight lifters. Ah, and before I forget, there are no muscles in the fingers, so you cannot build finger strength even if you want to. (xvimbi at this point will probably jump in to define “fingers”, but the metacarpals are not considered “fingers” in the common use of the word, and an anatomist would not go around talking about strengthening fingers when talking about the muscles of the “hand”, that is the metacarpals anyway).
Best wishes,
Bernhard.