Cat 6 should be more than adequate.
If you are only getting 40Mbs with that cable check the computer Network Interface Card(NIC) is set to 100/1000 and that the drivers are up to date.
If the NIC is good try a different cable.
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BT Smart Hub 2 with 70Mbs Download,18Mbs Upload, 9.17ms Latency & 0.35ms Jitter.