Swagger UI for net core 3.1 api is very slow

Question

I updated Our net core API application from 2.1 to 3.1, SwashBuckle.Asp.NetCore to 5.0.0. Here is my startup set:

public class Startup
{
    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    // This method gets called by the runtime. Use this method to add services to the container.
    public void ConfigureServices(IServiceCollection services)
    {
     string authServerUrl = "http://testserver.com/identityserver4";
         services.AddControllersWithViews();

        services.AddSwaggerGen(c =>
        {
            c.SwaggerDoc("v1", new OpenApiInfo { Version = "v1", Title = "NetCore API V1" });

            // Define the OAuth2.0 scheme that's in use (i.e. Implicit Flow)
            c.AddSecurityDefinition("oauth2", new OpenApiSecurityScheme
            {
                Type = SecuritySchemeType.OAuth2,
                Flows = new OpenApiOAuthFlows
                {
                    AuthorizationCode = new OpenApiOAuthFlow
                    {
                            AuthorizationUrl = new Uri(authServerUrl + "connect/authorize"),
                            TokenUrl = new Uri(authServerUrl + "connect/token"),
                            Scopes = new Dictionary<string, string>
                            {
                                { "netCoreAPI.read", "read permission" },
                                { "netCoreAPI.write", "write permission" }
                            }                        }
                }
            });

            c.AddSecurityRequirement(new OpenApiSecurityRequirement
            {
                {
                    new OpenApiSecurityScheme
                    {
                        Reference = new OpenApiReference { Type = ReferenceType.SecurityScheme, Id = "oauth2" }
                    },
                    new[] { "netCoreAPI.read", "netCoreAPI.write" }
                }
            });
        });
    }

    // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }

            app.UseRouting();

            app.UseAuthentication();

            app.UseAuthorization();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllers();
            });

            app.UseSwagger();
            app.UseSwaggerUI(c =>
            {
                c.SwaggerEndpoint("swagger/v1/swagger.json", "NetCore V1");
                c.EnableDeepLinking();
                c.OAuthClientId("clientId");
                c.OAuthClientSecret("clientSecret");
                c.OAuthAppName("netCoreApp");
                c.OAuthScopeSeparator(" ");
                c.OAuthUsePkce();
            });
        });
    }
}

The initial Swagger UI displays relatively quickly. However, when a method in a controller is clicked, it takes 30 seconds to display "Try it out" button. Is there a way to debug the problem? Or Is there anyone having the same problem? Before the code was converted from SwashBuckle 2.5 and net core 2.1 to SwashBuckle 5.0 and net core 3.1, the swagger UI works very fast.

Answer 1

I'm using "Swashbuckle.AspNetCore.SwaggerUI" Version="5.6.3" And with that version switching to "Swashbuckle.AspNetCore.Newtonsoft" was not really helping. There was no significant improvement.

Then I have fount this issue listed on Github. It is an old resolved issue but reopened in 2020. Where they explain Swagger UI 3.x has "Pretty print" and "Syntax highlight" which causing the render issues. It can be turned off in Swagger config:

SwaggerUI({
        syntaxHighlight: {
          activated: false,
          theme: "agate"
        },
        //url: path,
        ....
      });

In .NET Core you can access config as well: Setup.cs in Configure()

app.UseSwagger()
    .UseSwaggerUI(c =>
    {
        c.SwaggerEndpoint("/swagger/v1/swagger.json", "My Web API");
        c.ConfigObject.AdditionalItems.Add("syntaxHighlight", false); //Turns off syntax highlight which causing performance issues...
        c.ConfigObject.AdditionalItems.Add("theme", "agate"); //Reverts Swagger UI 2.x  theme which is simpler not much performance benefit...
    });

Answer 2

Are you using NewtonSoft? You need to add:

Install-Package Swashbuckle.AspNetCore.Newtonsoft -Version 5.1.0

And add:

services.AddSwaggerGenNewtonsoftSupport();
// explicit opt-in - needs to be placed after AddSwaggerGen()

https://github.com/domaindrivendev/Swashbuckle.AspNetCore#systemtextjson-stj-vs-newtonsoft