The ASA5520 by default has GigabitEthernet interfaces. So it can form a 1Gbps link with another device but as the 450Mbps throughput suggests, it cant fully utilize that link, even half of it.
The 50Mbps connection is most probably formed between the device ISP provides you and the ISP equipment at their datacenter. This naturally depends on actual media used to provide you the connection. It might even be a 100Mbps Fiber connection that has been set to 50Mbps on the ISP side through configurations.
One thing to notice since you mention 2 WAN connections. The ASA doesnt handle 2 WAN links particularly well. To my knowledge the only real supported form of using 2 WAN links with ASA is where the other WAN link simply acts as the backup and isnt used until the primary WAN fails.
Naturally there are ways to setup the ASA to utilize both of the WAN links.
For example virtualizing the ASA to Multiple Context Mode and letting 2 Security Contexts have their own WAN links. Or perhaps use special NAT configurations to manipulate your local LAN networks traffic to utilize both WAN links.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...