A couple of the CJ Dev team were down in Brighton for Re-Mix 2008 a few weeks ago, and one of the most useful sessions I went to was "ASP.NET Front End Performance" by Chris Hay.

There's some really obvious things, but others that I didn't know. I've collated the key points together for easy reference:

• Combine all JavaScript files together (i.e. single request)
• Combine all CSS files together (i.e. single request)
• Use inline scripts and CSS on the home page, load script and CSS assets post UI
• Reduce image requests
  • Use image maps
  • Use large background images
• Implement IIS caching
• Use versioning policy on filenames to avoid unwanted caching (e.g. my-js-file-001.js)
• Use Server.Transfer instead of Response.Redirect where possible
• Use SilverLight controls for common areas of the site (headers, footers, etc.)
• Implement HTTP compression in IIS
• Use the .NET ViewState wisely
  • Only on pages where necessary
  • Compress the ViewState
  • Offload ViewState to Session if possible / advantageous
• Avoid use of .NET UpdatePanel control where possible
• Expose web services to AJAX for use by JavaScript code, manually altering DOM
• Combine auto generated scripts by .NET framework (e.g. ScriptResource.axd)
  • Use script resource profiler to discover all script files
• Use SilverLight instead of AJAX
  • Detect presence of SilverLight, and fail safe to AJAX if preferred to forcing use of SilverLight
• Use Content Delivery Network where applicable (large projects) to bring the content closer to the user
• Implement use of data caching
• Use page / control output caching  
• Queue requests for processing (if any large / long requests will occur)
• Ensure all debug flags are removed from application

Cheers to Chris Hay!!