I bought a long usb cable and an adaptor for my Samsung Galaxy Note, but for some strange reason, while it indicates it charges my phone, the battery level rises veeeery slow
Many devices that you attempt to charge through a USB port on your PC will charge slowly unless the device is in sleep mode or turned off. Apple products are famous for this, but the limitation is due to USB power standards, so all high power usage devices can act similarly.
My advice is to turn the device off if charging through a PC. If you have a wall plug, turn the device off and use that, it should charge within a few hours.