When noticing speedtest.net results and large file downloads via the ProxySG appliance are slower than when accessing the internet directly.
The appliance negotiates the TCP Windows Size during the initial TCP handshake and retains the windows size throughout the session.
On the other hand, when accessing client traffic directly, the client has the Adaptive Window Sizing Support and is able to increase the TCP Window Size 'on the fly. during transmission, thus allowing more bytes.
Depending on your ProxySG model and capability, you can manually increase the initial TCP Window Size.
To manually increase the initial window size to 256K or higher:
#(config)tcp-ip window-size 256000
To revert the initial window size to default:
#(config)tcp-ip window-size 65535
Note: Enable Window Scaling, as described in RFC-1323; otherwise, the TCP Window Size remains at the default even after you increase it. Rrefer to KB article 000015466 for details on RFC-1323.