vs2017VSIX

[mirrortom]

I am using RazorEngine version 4.5. In the console program, compiling cshtml files into html is no problem.
Then, I added this program to the vs2017 VSIX plugin project, but found that the compilation time is very long, more than 18 minutes. I can't think of the reason. After trying, I found that adding this code to the configuration file can solve the slow compilation. But I don't understand why this is, please help me.
config.CompilerServiceFactory = new RazorEngine.Roslyn.RoslynCompilerServiceFactory();