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.

What's the size of your swagger.json file? Does the problematic endpoint use recursive and/or highly nested models? How many lines of JSON are in the displayed request bodies and response bodies? — Helen
@Helen I don't have swagger.json file. I am using Swagger in C# net Core Api to expose Api methods for Controller. When my application runs, it shows a web pages with all my controllers, and all the api methods for each controller. From there, I can test my api methods. My authorization server is IdentityServer4. — user3097695
See How to export a Swagger JSON/YAML file from Swagger UI. The slowness may be caused by the large file size and/or highly nested data models. — Helen
I have the same issue. The size of the swagger.json equals approximately 160kb — Calabonga

Steve Bates Steve Bates · Accepted Answer · 2020-03-12T11:17:19

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

Swagger UI for net core 3.1 api is very slow

4 Answers