We are currently checking the performance of our production sites (Windows 2003). They are serving a lot of HTTPS traffic, which we can't currently swap over to HTTP. I was wondering what were the best performance counters to investigate to see if our servers are being bogged down by the SSL connections?
Assuming you've already monitored your Internet connection to make sure it isn't saturated (that isn't SSL specific of course) the main impact of SSL is likely to be CPU, so use performance monitor to check the CPU over some suitable period. You may also want to monitor the process counter to check CPU usage for the inetinfo and w3wp processes.
I don't know of a way to drill down further, e.g. to see that it's SSL causing the high CPU as opposed to a rogue ASP script. I suppose you could run all SSL traffic in a separate application pool and just monitor CPU for the appropriate w3wp process.
If you want to use Windows performance counters the WWW service performance counter can give you a lot of useful information about a site or sites running in IIS (both http and https). You can find details of what you can log here.