its just a simple case of Maths
1A charger should take half as much time than a 500mA charger, your computer USB socket will be about 500mA, of course if the device is powered on then you need to deduct some mA for the device to power itself, 100mA unloaded sounds reasonable for that.
Device has a 2300mAh Battery, if your pumping 800mA in to it your looking at around 3 hours, 6 hours if its around 400mA
if your old adaptor was rated at 1A then its possibly faulty, but its also possible the device maybe restricting it, some devices wont draw its full amount if the COM cables are not connected, some chargers don't bridge the COM pins thus the phone thinks it isn't able to get its full load and may only draw a limited supply. Usual suspects for these chargers are those sealed ones with a cable attached.
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