From an initial search, it seems that the cisco SA520 should have a base throughput of 200mbps, but the VPN performance of the SA520 is supposed to top out at 65mbps however, so if you are running at 70, this may not be the issue.
(Though it is worth noting that a VPN on the SA540 - not the 520, but the 540 with High Performance would max out at 85mpbs throughput, and so 70mbps actual might make sense)
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In previous cases, it has been found that disabling IPS will increase performance considerably, as well as enabling auto port management in network settings and auto enabling ports on your ISP modem.
Your issue could also be related to your MTU sizes. While this value may vary based on ISP, the previous questioner found that a size of 1500 worked for them.
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Regarding the documentation here:
http://www.cisco.com/c/dam/en/us/products/collateral/security/small-business-sa500-series-security-appliances/at_a_glance_c45-562587.pdf
and the support forums question here:
https://supportforums.cisco.com/discussion/11489626/sa520-throughput-performance
