IPsec VPN - why are single thread network transfers very slow

I have a site to site VPN (IPsec) between a Mikrotik CCR1009 and Draytek 2960, both ends have 100Mb leased lines, latency is fairly low at 17ms, however:

A file move over SMB3 was slow at around 500KB/s (4Mb/s) with Windows 10 and iperf had similar performance with 1 session, however 20 concurrent sessions got well over 60Mb/s,

Is this just the impact of the latency on these protocols as they're not optimised for WAN levels of latency or am I missing something/hitting a bottleneck on my equipment?

Thanks,