We have a website which is sometimes performing slowly, and / or completely hangs. I have setted up temporarily the default system performance data collector in Performance Monitor, to see if this can shed some light.

However, the default Data Collector set collects a huge amount of counters, as well as generates huge logs files. Just 8 hours of data resulted in 4GB of data.

Which performance counters matter the most, when judging server load? Also, is it a performance concern if one leaves such data-collectors running indefinitely? Obviously, I will not know when the server will experience slow performance, so I need the logs there so that I can check them out. Any other specific guidelines on monitoring server performance would be greatly appreciated.

OS is a Windows Server 2008 R2 (Web Edition).