@TheGreatLeon
Dig speed goes from 100 to 120 to 140 “rate units”
No that is not how the dig speed bonus works. The official description "+20% faster artifact digging" is misleading. I would've interpreted the description in the same way as you did as well, but that's not the actual effect of the bonus.
People who chose the bonus figured out that the effect for choosing the bonus is multiplying dig times by 0.8, which is the same as saying that it increases the dig speed by 25% multiplicatively. That's not what you (or I) would have expected by reading the sentence "+20% faster artifact digging".
So the dig speed bonus is indeed exponential. It's way better than what the description suggests. I still wouldn't recommend getting it though lol