Shield already use AGY instead of STR, because of balance.
Shielders already move shield-hand too fast (like shield is a wrist watch), and just look at weight of some shields.
Just imagine how would this build look, that you need STR instead AGY for shield.
Also devs shod raise difficulty level for all weapons and armors. Look at Steel Shield - weight: 12.5 - difficulty: 6, really?
Transitional Armour - weight: 21.9 - difficulty: 14, yea right - so some agy spammer can run around in heavy armour.
Difficulty level need to be raised for all items for better balance.
There no need to make them faster, there lots of pretty much fast shield spammers in game right now. Shield + Scimitar = ultimate fast spam.
I give you + for making shielder exhausted from holding shield for long time and taking damage, but than we need stamina for all classes.