While it is certainly possible that your firewall is the bottleneck and some tuning as described in sk105119 posted by Gunter will help, dramatically bad performance through a VPN is likely to be an IP MTU/Fragmentation issue, see here: sk98074: MTU and Fragmentation Issues in IPsec VPN. Easy way to test if this is indeed the cause is to measure file transfer performance between the same two systems over the same network path, but NOT inside a IPSec tunnel (SCP/FTP/HTTPS/HTTP is fine for this test). If performance is substantially better outside the tunnel it is definitely a MTU/frag issue.
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