Signing and Encryption lower your performance
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Modern versions of SMB (especially 2.1 and later, and likely enforced in 3.1.1)
use a process called "signing" to verify the security of each data block transferred.
This often uses AES encryption which is computationally expensive without dedicated
cryptographic CPU instructions.  Pi4 and earlier (as with most RISC OS compatible
processors) lack such instructions, so there is an associated performance hit 
using signed transfers - often as much as 50%.

Fully encrypted connections will slow this even further.

If you have control over this (eg. many NAS devices allow you to enabled/disable
signing) then we suggest that you run unsigned for performance.  Alternatively, if
you have "guest" access enabled on your NAS, this often bypasses signing.

If you aren't sure, press menu over your LanMan98 shared disc icon on the iconbar
and go to Status->Share status and LanMan98 will tell you whether signing or
encryption is active.

During the LanMan98 beta phase we did a lot of work to boost AES performance,
and (depending on machine) throughput has almost doubled.  However, most of the
easy speedups are now implemented, so we suspect further boosts may be hard
to come by.  Synthetic Linux AES benchmarks on Pi hardware show similar drops,
so this is not a RISC OS-exclusive issue.