I am working with a List<> collection, adding new objects to the collection inside 2 nested loops. There are some 500000 items added to the collection, after the loops finish to execute.

At first, the addition operation runs well, but soon after there can be noticed a decrease in performance, for the last thousands of elements, the delay time is unbearable.

I have tried various tricks (initializing the collection with a certain size - 500000), replacing the List<> with a LinkedList<> collection, but it didn't help too much.

Can you recommend me a tip to solve the problem? I am interesting in changing the structure with a more optimized one - LinkedList<> for instance performs better than List<> with operations such as addition.

Method which updates the list

   private void UpdateForecastList(ConcurrentDictionary<Int32, RegistroSalidaProductoPrevision> prediccion, bool soloMejoresMetodos = true)
   {
        foreach (KeyValuePair<int, RegistroSalidaProductoPrevision> kvp in prediccion)
        {
            KeyValuePair<int, RegistroSalidaProductoPrevision> localKvp = kvp;

            IList<Prediccion> pExistente = prediccionList.Where(p => p.Id == localKvp.Key).ToList();

            Articulo articulo = (articuloList.Where(a => a.Id == localKvp.Key)).First();

            if (pExistente.Count > 0)
            {
                foreach (var p in pExistente)
                {
                    prediccionList.Remove(p);
                }
            }

            if (kvp.Value.Previsiones.Count > 0)
            {
                var previsiones = kvp.Value.Previsiones.Where(prevision => prevision.Value.LPrevision[1] != null).ToList();
                int previsionesCount = previsiones.Count;

                for (int a = 0; a < previsionesCount; a++)
                {
                    var registros = previsiones[a].Value.LPrevision[1].Serie;