How can I cache the master page in ASP.NET?
Unlike user controls, you can't OutputCache a Master page by itself--only as part of a Page.
Also, OutputCaching won't help the performance of a toolbar with lots of images anyway.
The kind of things that would help include image sprites, client-side caching, using a CDN, using multiple domains for static files, etc.
In case it's helpful, I cover those strategies in my book: Ultra-Fast ASP.NET.
If you use the @OutputCache directive on the page that uses the master page, then the master page will be cached along with the rest of the page.
来源：https://stackoverflow.com/questions/1871801/cache-master-page-in-asp-net