Well first important info you are not alone anymore. I test and got also 3 times faster transfer with smbclient than fstab. But the bad news sofar no idea why.
smb: \> get test.file
getting file \test.file of size 681867016 as test.file (7080.1 KiloBytes/sec) (average 6734.2 KiloBytes/sec)
osmc@osmc-vero4k:~$ dd if=/mnt/server.my.hk/test.file of=/home/osmc/test.file bs=1M count=3072650+1 records in
650+1 records out
681867016 bytes (682 MB, 650 MiB) copied, 249.569 s, 2.7 MB/s