Warnings and errors will be send to the windows event log.
IISpeed exposes a few urls that allow viewing diagnostic information. By default, these urls are only accessible from the local machine. If you want them to be publically accessible, or authorize them yourself using the request filtering module, you can add this to IISWebSpeed.config:
pagespeed InfoUrlsLocalOnly off
Viewing the last few IISpeed log messages
These messages are collected globally from all worker processes.
PageSpeed Global Statistics
Global statistics and configuration can be viewed at
PageSpeed WebSite Statistics
Per website statistics and configuration can be viewed at
The PageSpeed console also offers opportunities for monitoring your IISpeed installation.
IISpeed debug output can be viewed using DebugView (downloadable at microsoft technet).
To get as much debug information as possible, you can also put a line containing
pagespeed diagnose into the root configuration file.
If you're capturing diagnostics of IISpeed via DebugView, make sure to enable
Capture Global Win32.