How to reduce time of instantiating a prefab that contains many objects/components in it.

This is going to sound a bit insane, and I don't use it any more because I have since reduced the texture data size from 1 GB to 0.1 GB, so it is not a problem now. But I used this in the Menu scene before the user started a level (which load the Game scene and access the "sharedassets1.assets.resS" data file).

I preloaded the resource file into the system cache by reading the file in a thread like this code here, and it worked, it reduced the loading time by several seconds.

 using System.IO;
 using System.Threading;
 
 //to avoid a 6 second freeze when the file is not in cache
 // this is loaded here once hopefully while the user spend some seconds in the menu
 // before this will be accessed by LoadSceneAsync.
 byte[] preLoadBytes = new byte[1024 * 1024]; //1MB buffer
 string preLoadDataPath;
 Thread preLoadThread;
 void PreLoadAssetsToCache()
 {
     bool allRead = false;
     int iChunkSize = preLoadBytes.Length;
     int iChunkNr = 0;
     FileStream fs = null;
     try
     {
         fs = File.OpenRead(preLoadDataPath + "/sharedassets1.assets.resS");
     } catch { }
     while (fs != null && !allRead)
     {
         int fr = fs.Read(preLoadBytes, 0 + iChunkNr * iChunkSize, iChunkSize);
         if (fr != iChunkSize) allRead = true;
         iChunkNr++;
         Thread.Sleep(5);
     }
 }
 
 //done at the first state in the Update() state machine
 //to avoid stalls of 6+ sec the first time a level is started after app start
 preLoadDataPath = UnityEngine.Application.dataPath;
 ThreadStart ts = new ThreadStart(PreLoadAssetsToCache);
 preLoadThread = new Thread(ts);
 preLoadThread.Priority = System.Threading.ThreadPriority.Lowest;
 preLoadThread.Start();

You can try it, hope it will work. But there must be some idle time before you want to load the data in Unity for it to not instead increase the workload. This is most useful on a PC, don't think it will work well on Android or similar.