In addition to Alonzo's remarks in checking duplex etc.. I would suggest you gather basline overall traffic/connections information from the ASA firewall.. I have 515E's and I do use the following link for keeping eyes opened on performance issues when they appear, although the bellow link pertains for PIX 500 series most of the information in it can be used as an example in gathering info & troubleshooting performance on the ASA as well.
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...