Re: galaxy s4 using while charging
Of course it is. The phone draws a fixed amount of current (as long as the charger can supply it - USB charging usually can't, so the phone charges slower). If the phone is off, all of that current goes to the battery. If the phone is running, part of the current goes to the battery and part runs the phone. (Or the battery is being discharged by the phone at a slower rate than it's being charged by the charger - electrically and mathematically, there's no difference how you phrase it [Kirchoff's Law]) - so if the phone is on it's going to charge slower.
The only way to avoid that would be to allow the phone to draw as much current as the battery charging and maximum phone current required - and if the phone was off or wasn't using its maximum current, that would damage the battery, so it's not done.
So, yes, bottom line - the phone will charge slower when it's being used than when it's not being used.E
Edit:
1% over 10 minutes, though, is a bit slow. That's 6%/hour. That's a 0.16C charge, which is very slow. (Normal charge is about 0.75C on a portable charger.) Either you're doing some heavy lifting (an intensive game, viewing videos full screen wih the brightness all the way up, that sort of thing), you're charging from a computer (remember, USB charging doesn't supply as much current) or there's something wrong with the charger, the cable, the phone or the battery is on its way out (which can happen in less than a year if the battery isn't treated properly).
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