We have SD-WAN and use SMB (but v2) and don't have problems. Our HQ has a 500/500 uncontended link and the DataCentre has a 1000/1000 uncontended link on the same carrier, however these are L2 MPLS (VPLS) and not DIA.
I regularly get 450Mb+ file transfers between the sites. MXs are MX250 at DC and MX100 at HQ, all running 18.107.10, though they have been fine from 15.x upwards.
Are you testing at times of low activity and if so what is the utilization on the MXs at this time?
What MX versions are you running?
What models are at each end (I think you mentioned this in another thread?)
If you do an internet speedtest from each site, what do you get?
Are they on the same ISP (as I believe you have DIA and not private WAN?
How have you verified this: (verified both are getting expected speeds)?
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