slow connection with usb port in P-660RU-t1 v3s

today i bought a P-660RU-t1 v3s modem. this model has two port: one LAN and one USB. when i use LAN everything is OK and i have maximum speed(16 mb/s) but when i use usb port my speed become about 8mb/s. i dont know why. i installed usb driver for windows 7 (last version in CD) but my windows is 10.
can anyone help?

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