Most efficient way to create/spawn a lot of enemies at the same time(Unity3D)

I googled a lot and most EnemySpawners I found online only spawn one Enemy after another. In my game I want to spawn hordes of enemies in timed waves. So I did it the best way I could think of, which is the code snippet below. Around the code snippet there is a system that checks if it is time to spawn enemies. The "yield return new WaitForEndOfFrame();" is to make sure, that not every enemy is spawning at the same time, so the game is not freezing because of it, it is still very slow though. Is there a more efficient way? The enemies are prefabs that I instantiate and they have a Skinned Mesh Renderer. If you need more Information, I am happy to provide them. Thanks in advance

             float currentSpawnZ = ...;
             foreach (EnemySpawn enemySpawn in nextSpawnGroup.enemySpawns) {
                 List<GameObject> enemyList = new List<GameObject>();
                 float currentSpawnX = ...;
                 for (int i = 0; i < enemySpawn.amount; i++) 
                 {
                     GameObject enemy = Instantiate(enemySpawn.enemy, hierarchyParent, true);
                     enemy.transform.position = new Vector3(currentSpawnX, ... , currentSpawnZ);
                     enemyList.Add(enemy);
                     currentSpawnX += spawnXGap;
                     yield return new WaitForEndOfFrame();
                 }
                 currentSpawnZ += spawnZGap;
                 enemiesSpawnedEvent.Invoke(enemyList);
             }