Funny that you should mention any of this. Let me give a 5000' view of the issue.
Batteries are generally shown with a charging curve. That is to say that your discharged battery isn't at 0% charge, the potential between the terminals is just too low to create the required voltage. Electronics actually work with a voltage regulator, that takes some input value and generally only allows a certain amount through. That is to say you may have a set of 10 serial cells that produce 12 volts, but the regulator onboard provides 11 volts to run the system. That's cool, because as the potential drops those cells go from a potential of 1.2 volts each to 1.1 volts, and at 11 volts your device still runs.
Thus, your battery life, in percentage is usually something like a measurement of the voltage being put out by the battery, against a model of the fall-off between peak and minimum voltage. That is to say 12 volts = 100%, 11.4 volts = 50%, 11.1 volts = 25%, and 10.9 volts = 0%. Notice that's not a linear relationship, because as you get more energy stored in the battery the potential increases, increasing the resistance to charging. This is where that cell charging curve, mentioned above, comes in.
What is "slow charging" then? Old school definition is to limit amperage and have a constant voltage. This means 0-10% is way faster than 90%-100%, but the lower rate decreases heat build-up and formation of solid crystals inside the material which decrease charge capacity. It also decreases heat, lowering the likelihood that that LI-ion cell starts to off-gas and decrease capacity. The 85% thing is a rule-of-thumb, where generally the charge curve at that point is basically so high that you don't have to build that excess heat and can still "fast charge" the cells without damage.
It's kind of funny. Apple, a few years back, touted fast charging as a panacea. That logic made exploding cells a reality for badly designed electronics, and now people are starting to realize that plugging that wall-wart in over night is actually leading to less wear and tear than that expensive fast charger...too bad Apple designs disposable product. Anger aside, it's funny that people are only now realizing this. Lead-acid batteries have this same process...and they've been around for decades (commercially). They've been around for centuries as known commodities.