DBContext and Repository Pattern run slowly

in my solution there are two projects: a .net core class library named "Infrastructure" and a .net core web application (mvc) named "Admin". In the "Infrastructure" prj I've implemented the Repository Pattern for Application DBContext like:

public partial class ApplicationDbContext : DbContext
{
   ....
}

Interfaces:

public interface IGenericRepository<T> where T : class
{
    T Add(T t);
    ....
}

public interface ICustomersRepository : IGenericRepository<Customers>
{
    Customers GetCustomerByCompanyCode(string companyCode);
    void InsertOrUpdate(Customers toSync);
}

Generic:

public class GenericRepository<T> : IGenericRepository<T>, where T : class
{
    protected ApplicationDbContext _context;

    public GenericRepository(ApplicationDbContext context)
    {
        _context = context;
    }

    public virtual T Add(T t)
    {
        _context.Set<T>().Add(t);
        _context.SaveChanges();
        return t;
    }

    ....
}

Specific:

public class CustomersRepository : GenericRepository<Customers>, ICustomersRepository
{
    public CustomersRepository(ApplicationDbContext context) : base(context)
    {
    }
    ....
}

In the Web Application: - Startup.cs

public void ConfigureServices(IServiceCollection services)
{
    services.AddDbContext<ApplicationDbContext>(options => options.UseNpgsql(Configuration.GetConnectionString("DefaultConnection"))); 
    ....
    services.AddScoped(typeof(IGenericRepository<>), typeof(GenericRepository<>));
    services.AddScoped<ICustomersRepository, CustomersRepository>();
    ....
}

- CustomersController.cs

private readonly ICustomersRepository rep;

public CustomersController(ICustomersRepository rep, ....)
{
    this.rep = rep;
    ....    
}

public async Task DoImport()
{
    ....
    rep.InsertOrUpdate(mapper.Map<Customers>(item.Result));
    ....
}

It works but very slowly: 300.000 insert in few hours. If I don't use Repository Pattern but try to insert calling ApplicationDBContext directly like:

using (var _context = new arval4proContext())
{
     _context.Add(toSync);
     _context.SaveChanges();
}

the work time is: 300.000 insert in 6 minutes.

Why is this happening? Is the Repository Pattern the best choice to separate Data Access Layer in .net core projects? There are other solutions to do this? Tnx in advance.