What I've learned is that raw code on a page, with no database connection, will execute in php in about 0.15 seconds, and in .NET in about 0.015 seconds. An eighth of a second isn't much, but it's consistent.
The biggest issues I had in converting the site to .NET was converting the database from MySQL to MS SQL. It required about a hundred specially formatted .CSV files and the original MySQL database had a few corrupt tables which couldn't be repaired.
Most of the pages in the new .NET site are actually built in the code-behind pages using stringbuilders to concatenate raw HTML. This allows for better scaling than anything else I've tried, and because of my experience in Classic ASP I find it easy to code.