Nexus Charging VERY slowly.

Normal charge time from fully discharged should be about 16 hours, charger or USB, with the phone off. If something in your computer is limiting the available USB current, it'll charge more slowly in the computer.

As for how long it takes to charge when you're using it, that depends on what you're doing with it (IOW, how much current you're making it draw). Remember, the microUSB port is a charging port, not an external power port. It's very easy on most phones to draw more current than the charger can supply, and run the battery out of charge even while it's plugged in.

As far as your iPhone went, that's not a USB connector, it's a proprietary Apple port, which can probably handle a lot more current than a USB 2.0 port.