If you are talking about lithium ion batteries, the last 20% is slower due to recharge circuitry programming. Initially, the battery is in a constant-current charge sequence where it might be charging at a high rate (a rate that would charge the entire capacity of the battery in 1 hr). As the battery charges, the voltage rises. Once it reaches a pre-programmed maximum voltage the system switches to constant-voltage charging, variable current (see the plateau in the charge curve below). The cell demands less and less current as the battery reaches a fully charged state. By the end it is demanding only a few milliamps. It could go faster if the charger would increase the charging voltage, but this can cause bad things to happen and shorten the cycle life of the battery.
Chemically, the lithium ions are intercalating into the graphite or LTO anode and they must diffuse deeper and deeper into the sheets or crystal, respectively, to utilize most of the anode material. The potential (voltage) difference between anode and cathode rises as the anode becomes saturated with lithium requiring either more voltage to speed it up or longer periods of time at safer, lower voltages.