Infusion enchantment essentia costs are known to scale exponentially with both the number of unique infusions (y= c * 1.33^x) and the level of a particular infusion (for runic shielding, y = 20 * 2^x + 40 in total.) Particularly for the latter, at high levels the jump in infusion cost rapidly and dramatically outscales the effect of the cost infusion stone (at the time of writing, I have been waiting on one infusion for about 3 hours.) While one could simply use the config to vastly increase the cost reduction, this comes at the price of wreaking havoc on lower-level infusion balance.

As such, it would be nice to see a variant of the Cost Infusion Stone whose cost reduction effect scales directly with the base infusion cost - for example, with one infusion stone, for every a points of essentia the infusion would cost at base, the infusion stone decreases the final cost by b%, down to a minimum of c% of the base cost - with a, b, and c all being configurable. Whether or not multiple stones stack additively or multiplicatively could also be configurable, although this last bit is a little more dubious.

I don't actively mod for minecraft nowadays, but even if I did, I would not implement your suggestion because the exact purpose of the exponential cost of the runic shielding enchantment is to prevent a player from getting too much of it. If you had to spend 3 hours infusing then it means you should stop already, this is a realistical maximum which you should not push further if you value your time.

To make things a bit more clear, if I added such cost stone variant to the game then it would simply increase the realistically maximum amount of runic shielding by some constant number (something close to log2(1/c)), and I think it is entirely pointless.

Besides that, if your main concern is infusion time rather than cost then you should be using acceleration stones instead of cost reduction stones. Combined with Thaumic Augmentation's speedy matrix you can achieve the essentia absorption rate of 20 per second.