Smartphones and tablets generally will not charge at their maximum rate without company/model specific knowledge about the charger they are connected to.
USB charging from a generic Vbus +5vdc supply from a computer USB port or generic USB charger is often limited to less then 100 mA. This is due to the fact that most smartphones rely on the charger to perform the current limiting to the phone's required maximum charge rate based on size of cellphone's battery. The phone must be sure it is connected to its proper (usually company unique) charger because the phone is relying on charger to limit the charge current to about 0.8 times the Li-Ion capacity, mAH rate, in mA's.
If the phone does not get the proper detection that it is its proper matching charger then it reduces its charge rate to avoid overloading the computer/laptop USB port, and more important to the phone, to avoid the linear regulator in the phone from overheating. By letting the charger do the maximum charge rate current limiting, the phone's linear regulator can go into full saturation bypass mode thereby reducing the IR heating loss in the phone.
Although USB 1 and USB 2 specs allow for up to 500 mA after negotiation from initial 100 mA, many phone manufacturers find this too complicated and just limit the charge current to less then 100 mA from an unidentified charger.
When the smartphone sees Vbus of 5vdc but does not get the proper detection of its unique charger then the phone's charge regulator goes into linear mode, regulating charge current to a lower level to avoid overheating in phone. It can take much longer to recharge under this condition.
USB 1 and USB 2 standard allow up to 500 mA draw from USB Vbus after negotiation from the initial 100 mA limit. There is total crazy variations on how particular manufactures adhere to official USB specs on Vbus loading. USB 3 standard has tried to bring some consistency to this and allow for higher then 500 mA loading but it will take some time, if ever, for manufactures to comply. Some laptops, for example will totally shut down their Vbus sourcing if a connected device tries to draw more then 100 mA without proper USB negotiation. Some laptops will only allow a maximum of 100 mA draw to conserve their own batteries.