Adjusting the window size is not a bad idea. Here are the defaults below:
sysctl -a | grep space | grep tcp
I would suggest increasing and testing again. Some people choose to double the value 32768 -> 65536.
Also keep in mind that increasing this value requires the firewall to temporarily store more data, so if you have many connections, it may hog the resources of the appliance.
Let us know how it goes.
I am sorry, I forgot to give you the command to increase it:
Also, if you want it to be persistent after reboot, please add to /etc/sysctl.conf.