Under Security Appliance > Configure > Traffic shaping, what is your Uplink configuration set to? If you have it set to the full 2 Gbps, your device could be trying to pull bandwidth that it is incapable of doing. You should set this to the actual speeds you are paying for (I learned this from support a few months back).
Additionally, there is a section for Global bandwidth limits which sets a per-client limit. What is that set to? This could be affecting your speed if it is set too low.
And of course, like everyone is saying, check the duplex setting on the uplink connection.
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