An explanation of how the ProxySG distributes traffic to multiple interfaces using LACP:
LACP assigns packets to physical links using a hash function and does not guarantee a perfectly even distribution of traffic among all the links. Hash function is done on source MAC, destination MAC, VLAN tag, source IP, and destination IP. Packets for any given connection will always be transmitted over the same physical link, limiting the burst speed for the connection to the capacity of one link rather than the sum of all the links.
At the time of this writing, this behavior cannot be changed.