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SMB was not designed to be used over a WAN.
For example if you are using SMB version 1 or 2 the maximum block size is 64 kb. With a Round Trip Time (RTT) of 60 msec the maximum throughput will be 8,74 Mbps, regardless of the higher available bandwidth.

But when a 500 MiB file takes 2 hours then the average speed is 622 kbps. So bandwidth is not the issue (unless the WAN link is saturated with other traffic).

I noticed was a very signification number of dup-acks, probably few thousand in this particular pcap file.

A few thousands is still be less than 1 percent packets on a 500 MiB file. Anyway, check if TCP SACK is enabled (and used) and Window scaling is enabled/used in case of SMBv3.

But it is better to switch to a protocol that performs well over a WAN. Like (S)FTP, FTPS, HTTP(S).