If you're not going to use an ORM, are you also not going to cache your data? That's a big advantage of using an ORM. If there's no data cache, you'll need to look at ways to cache the HTML/JavaScript. This can be accomplished using OutputCache directive and SqlDependency. Or by publishing out static HTML and JavaScript files. Either way, you will be able to handle higher load if you're not constantly hitting the database on every request.
Some links:
ASP.NET Web Site Performance Improvement
http://www.codeproject.com/KB/aspnet/aspnetPerformance.aspx
10 ASP.NET Performance and Scalability Secrets
http://www.codeproject.com/KB/aspnet/10ASPNetPerformance.aspx
Omar AL Zabir blog on ASP.NET Ajax and .NET 3.5
http://msmvps.com/blogs/omar/archive/tags/performance/default.aspx
