Performance of F5 BIG IP

f5 have different types of virtual server according to requirements, as well as different hardware to scale onto. For instance, a standard HTTP virtual server acts as a full proxy and allows clever layer 7 tweaks to be done such as changing URLs or very whizzy things. However, a Performance HTTP or FastLayer4 type is not full proxy and reduces the amount of layer 7 functionality but is faster. f5 also has hardware packet acceleration and SSL encryption hardware offload to enable fast packet transfer.

However, I don't have any specifics about supported configurations on different hardware but my thoughts are that most f5 boxes would handle 10K Exchange clients with no problems. Have you seen the Exchange iApp? It massively simplifies configuration. SOL8365 - Deploying F5 with Microsoft Exchange Server