.NET CORE 3 With Angular 9 loading extremely slowly

I've bought the book ASP.NET CORE 3 and Angular 9 third edition and am currently reading it and coding along. I do however have a problem, when I launch the app (pressing F5 from visual studio 2019) every request to the server is extremely slow (it can take 25 seconds sometimes, and The amount of data is not that big, 225 countries and I'm fetching them 10 by 10) and I don't know why.

I don't know exactly what file I should show you guys, so if you have any ideas or want some files details, please tell me in the comments.

I do have that message in the console:

fail: Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware[1]
      An unhandled exception has occurred while executing the request.
Microsoft.Data.SqlClient.SqlException (0x80131904): Execution Timeout Expired.  The timeout period elapsed prior to completion of the operation or the server is not responding.
 ---> System.ComponentModel.Win32Exception (258): Dépassement du délai d'attente.
   at Microsoft.Data.SqlClient.SqlCommand.<>c.<ExecuteDbDataReaderAsync>b__164_0(Task`1 result)
   at System.Threading.Tasks.ContinuationResultTaskFromResultTask`2.InnerInvoke()
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
--- End of stack trace from previous location where exception was thrown ---
   at System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task& currentTaskSlot, Thread threadPoolThread)
--- End of stack trace from previous location where exception was thrown ---
   at Microsoft.EntityFrameworkCore.Storage.RelationalCommand.ExecuteReaderAsync(RelationalCommandParameterObject parameterObject, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Storage.RelationalCommand.ExecuteReaderAsync(RelationalCommandParameterObject parameterObject, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Storage.RelationalCommand.ExecuteReaderAsync(RelationalCommandParameterObject parameterObject, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryingEnumerable`1.AsyncEnumerator.InitializeReaderAsync(DbContext _, Boolean result, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.SqlServer.Storage.Internal.SqlServerExecutionStrategy.ExecuteAsync[TState,TResult](TState state, Func`4 operation, Func`4 verifySucceeded, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryingEnumerable`1.AsyncEnumerator.MoveNextAsync()
   at Microsoft.EntityFrameworkCore.EntityFrameworkQueryableExtensions.ToListAsync[TSource](IQueryable`1 source, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.EntityFrameworkQueryableExtensions.ToListAsync[TSource](IQueryable`1 source, CancellationToken cancellationToken)
   at WorldCities.Data.ApiResult`1.CreateAsync(IQueryable`1 source, Int32 pageIndex, Int32 pageSize, String sortColumn, String sortOrder, String filterColumn, String filterQuery) in E:\Software development\Portfolio\Projects\WorldCities\Data\ApiResult.cs:line 78
   at WorldCities.Controllers.CitiesController.GetCities(Int32 pageIndex, Int32 pageSize, String sortColumn, String sortOrder, String filterColumn, String filterQuery) in E:\Software development\Portfolio\Projects\WorldCities\Controllers\CitiesController.cs:line 33
   at lambda_method(Closure , Object )
   at Microsoft.Extensions.Internal.ObjectMethodExecutorAwaitable.Awaiter.GetResult()
   at Microsoft.AspNetCore.Mvc.Infrastructure.ActionMethodExecutor.AwaitableObjectResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeActionMethodAsync>g__Awaited|12_0(ControllerActionInvoker invoker, ValueTask`1 actionResultValueTask)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeNextActionFilterAsync>g__Awaited|10_0(ControllerActionInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Rethrow(ActionExecutedContextSealed context)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeInnerFilterAsync>g__Awaited|13_0(ControllerActionInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeNextResourceFilter>g__Awaited|24_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Rethrow(ResourceExecutedContextSealed context)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeFilterPipelineAsync>g__Awaited|19_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeAsync>g__Awaited|17_0(ResourceInvoker invoker, Task task, IDisposable scope)
   at Microsoft.AspNetCore.Routing.EndpointMiddleware.<Invoke>g__AwaitRequestTask|6_0(Endpoint endpoint, Task requestTask, ILogger logger)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)
ClientConnectionId:e2d0fb93-033c-4ec3-8275-ef4ce243b670
Error Number:-2,State:0,Class:11

here is the query:

public async Task<ActionResult<ApiResult<City>>> GetCities(
            int pageIndex = 0,
            int pageSize = 10,
            string sortColumn = null,
            string sortOrder = null,
            string filterColumn = null,
            string filterQuery = null)
        {
            return await ApiResult<City>
                .CreateAsync(
                    _context.Cities,
                    pageIndex,
                    pageSize,
                    sortColumn,
                    sortOrder,
                    filterColumn,
                    filterQuery);
        }

/// <summary>
        /// Pages, sorts and/or filters a IQueryable source.
        /// </summary>
        /// <param name="source">An IQueryable source of generic type</param>
        /// <param name="pageIndex">Zero-based current page index (0 = first page)</param>
        /// <param name="pageSize">The actual size of each page</param>
        /// <param name="sortColumn">The sorting colum name</param>
        /// <param name="sortOrder">The sorting order ("ASC" or "DESC")</param>
        /// <param name="filterColumn">The filtering column name</param>
        /// <param name="filterQuery">The filtering query (value to lookup)</param>
        /// <returns>
        /// A object containing the IQueryable paged/sorted/filtered result 
        /// and all the relevant paging/sorting/filtering navigation info.
        /// </returns>
        public static async Task<ApiResult<T>> CreateAsync(
            IQueryable<T> source,
            int pageIndex,
            int pageSize,
            string sortColumn = null,
            string sortOrder = null,
            string filterColumn = null,
            string filterQuery = null)
        {

            if (!String.IsNullOrEmpty(filterColumn) && !String.IsNullOrEmpty(filterQuery) && IsValidProperty(filterColumn))
            {
                source = source.Where(String.Format("{0}.Contains(@0)", filterColumn), filterQuery);
            }

            var count = await source.CountAsync();

            if (!string.IsNullOrEmpty(sortColumn) && IsValidProperty(sortColumn))
            {
                sortOrder = !String.IsNullOrEmpty(sortOrder) && sortOrder.ToUpper() == "ASC" ? "ASC" : "DESC";
                source = source.OrderBy(string.Format("{0} {1}", sortColumn, sortOrder));
            }

            source = source.Skip(pageIndex * pageSize).Take(pageSize);

            var data = await source.ToListAsync();

            return new ApiResult<T>(data, count, pageIndex, pageSize, sortColumn, sortOrder, filterColumn, filterQuery);
        }

When I enter the URL to get the data directly in my web browser, I get the result immediatly.

Thanks a lot.

I've debugged the app and retrieved the SQL generated:

SELECT [c].[Id], [c].[Name], [c].[Lat], [c].[Lon], [c0].[Id] AS [CountryId], [c0].[Name] AS [CountryName]
FROM [Cities] AS [c]
INNER JOIN [Countries] AS [c0] ON [c].[CountryId] = [c0].[Id]
ORDER BY [c].[Name] DESC
OFFSET @__p_0 ROWS FETCH NEXT @__p_1 ROWS ONLY  

I think it comes from this line of code:

source = source.Skip(pageIndex * pageSize).Take(pageSize);

the error I get from SSMS when I try the generated query is the following:

Msg 137, Level 15, State 2, Line 5
Must declare the scalar variable "@__p_0".
Msg 137, Level 15, State 2, Line 5
Must declare the scalar variable "@__p_1".