Slow first page load when porting asp.net core from .net 5 to .net 6

I have ported my application from asp.net core 5.0 to asp.net core 6.0 (following the recommended steps but keeping the .net core 5 hosting model) and I'm having a problem in dev: the initial load time of pages in the new version is very slow.

I have more or less pitpointed the issue to the use of Razor Runtime compilation:

        IMvcBuilder mvcBuilder = services
            .AddControllersWithViews(
                configure =>
                {
                    configure.InputFormatters.Insert(0, new RawJsonBodyInputFormatter()); // this allow receiving a JSON object as a string
                    configure.Filters.Add<PeppermintExceptionsFilter>();    //new UnauthorizedExceptionFilter());
                })
            .AddViewLocalization(Microsoft.AspNetCore.Mvc.Razor.LanguageViewLocationExpanderFormat.Suffix)
            .AddDataAnnotationsLocalization(option =>
            {
                option.DataAnnotationLocalizerProvider = (type, factory) => factory.Create(typeof(SharedResource));
            })
            .AddJsonOptions(options =>
            {
                options.JsonSerializerOptions.PropertyNamingPolicy = null;
            })
            .AddFluentValidation();

        if (environment.IsDevelopment())
        {
            mvcBuilder.AddRazorRuntimeCompilation(); // commenting out this line resolves the issue
        }

When the line is present, initial load for all pages takes up to 30 seconds (typically, the first page takes 30 seconds and other pages are often quicker).

If I comment out the option, then everything works fine again. This is clearly not a problem in production but it's really problematic (and frustrating) for developpement.

The problem is not present in asp.net core 5.0

Adding the AddRazorRuntimeCompilation option to a new .net 6.0 using the new hosting model) application seems to induce the same issue but I'm not sure because the compilation of pages in a nearly empty application is very quick.