Hello,
Your speeds are most likely slower because of the security services running on your ISA.
I took a quick look at the datasheet for the ISA550 and the throughput when the various threat management features are enabled is listed at 45Mbps, right around what you are getting.
As a test go through and disable the security services one at a time to narrow down which one is slowing you down, my guess would be the A/V or IPS.
So it is fixable in a way, using just the firewall the device can handle up to 200Mpbs, but that does mean you would need to disable some security features.
Hope that helps and thank you for choosing Cisco,
Christopher Ebert - Advanced Network Support Engineer
Cisco Small Business Support Center
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