Really Poor SMB performance

Fortinet to Fortinet, 100E to 60E, IPSec Tunnel, gigabit connection on the 100E and 400mbit on the 60E.

SMB transfers are slow, about 2 or 3mbps.

Have adjusted tcp-mss in the IPV4 policy for the indicated branch and on the IPSEC interface itself to 1306 (which is low but higher doesn't matter, when left at default it was fragmenting so I lowered it)

config sys interface

edit <interfacename>

set tcp-mss 1306

end

AND

config firewall policy

edit <policy number>

set tcp-mss-sender 1306

set tcp-mss-receiver 1306

end

(configured both legs of the firewall policy, inbound and outbound, on both firewalls)

Perhaps of note, IPSEC tunnels to Juniper firewalls perform normally (also have tcp-mss set to 1306) . . .

What setting am I missing?