A script to improve performance of RealtimeTransforms in Rigidbody mode. See comments for fixes required for the latest Normcore versions.

RealtimeTransformOptimizer.cs

using Normal.Realtime;
using System;
using System.Collections;
using System.Reflection;
using UnityEngine;

namespace Normcore.Realtime
{
	/// <summary>
	/// This script is intended to be attached to an object with a RealtimeTransform in Rigidbody mode. We have to use reflection 
	/// to override the behaviour because the relevant normcore methods/classes are private/internal/sealed/etc.
	/// 
	/// RealtimeTransform's FixedUpdate method when using the Rigidbody strategy performs some operations multiple
	/// times instead of caching and re-using the results. For one object its a small amount, but when you have dozens
	/// of objects and multiple fixed updates running in one frame it can add up quickly. 
	/// 
	/// While profiling i saw that I had 83 realtime transforms with rigidbodies and on average 2 fixed updates per frame. 
	/// The default behaviour accesses RealtimeModel isOwnedLocallySelf and ownerIdSelf twice each, which is already 332 calls.
	/// This was taking .76ms on my high-end development PC. I didn't test on quest, because its already too high.
	/// 
	/// I added this script to some of the realtime transforms in the scene to roughly compare values in the profiler. 
	/// In a frame with three fixed updates, 30 instances using the original RealtimeTransform took 0.37ms to complete.
	/// In that same frame, 54 instances with this script added took 0.21ms to complete. It's a small sample size and imprecise
	/// metrics, but thats roughly 3 or more times faster if you scale to the same quantity. It would likely be faster again if
	/// this was using normcore methods directly instead of using reflection.
	/// </summary>
	[DefaultExecutionOrder(-94)] // One higher than RealtimeTransform
	public class RealtimeTransformOptimizer : MonoBehaviour
	{
		[SerializeField] RealtimeTransform _realtimeTransform;
		[SerializeField] RealtimeView _realtimeView;
		[SerializeField] Rigidbody _rigidbody;

		bool _isInitialized;
		int _clientId = -1;
		bool _isAlreadyOwned;
		RealtimeTransformModel _realtimeTransformModel;

		Action _incrementFixedRoomTime;
		Action<RealtimeTransformModel> _remoteFixedUpdate;

		static bool _isStaticInitialized;
		static FieldInfo _strategyField;
		static PropertyInfo _modelProperty;
		static MethodInfo _incrementMethod;
		static MethodInfo _remoteFixedUpdateMethod;

		void Reset()
		{
			_realtimeTransform = GetComponent<RealtimeTransform>();
			_realtimeView = GetComponent<RealtimeView>();
			_rigidbody = GetComponent<Rigidbody>();
		}

		void OnEnable()
		{
			if (_realtimeTransform == null) {
				enabled = false; // Support stripping realtime components at runtime
			} else {
				_realtimeTransform.StopAllCoroutines();
			}
		}

		void Awake()
		{
			_realtimeView.didReplaceAllComponentModels += HandleReplacedAllComponentModels;
		}

		void OnDestroy()
		{
			if (_realtimeView != null) {
				_realtimeView.didReplaceAllComponentModels -= HandleReplacedAllComponentModels;
			}

			if (_realtimeTransform != null && _realtimeTransform.realtime != null) {
				_realtimeTransform.realtime.didConnectToRoom -= HandleConnected;
				_realtimeTransform.realtime.didDisconnectFromRoom -= HandleDisconnected;
			}
		}

		void FixedUpdate()
		{
			if (!_isInitialized || _clientId == -1) {
				return;
			}

			_incrementFixedRoomTime();

			if (_realtimeTransform.ownerIDSelf == _clientId) {
				if (!_isAlreadyOwned) {
					_rigidbody.WakeUp();
					_isAlreadyOwned = true;
				}
			} else {
				_remoteFixedUpdate(_realtimeTransformModel);
				_isAlreadyOwned = false;
			}
		}

		void HandleConnected(Normal.Realtime.Realtime realtime)
		{
			_clientId = realtime.clientID;
		}

		void HandleDisconnected(Normal.Realtime.Realtime realtime)
		{
			_clientId = -1;
		}

		void HandleReplacedAllComponentModels(RealtimeView view)
		{
			object strategyValue;

			if (!_isStaticInitialized) {
				_strategyField = _realtimeTransform.GetType().GetField("_strategy", BindingFlags.Instance | BindingFlags.NonPublic);
				_modelProperty = _realtimeTransform.GetType().GetProperty("model", BindingFlags.Instance | BindingFlags.NonPublic);

				strategyValue = _strategyField.GetValue(_realtimeTransform);

				_incrementMethod = strategyValue.GetType().GetMethod("IncrementFixedRoomTime", BindingFlags.Instance | BindingFlags.NonPublic);
				_remoteFixedUpdateMethod = strategyValue.GetType().GetMethod("RemoteFixedUpdate", BindingFlags.Instance | BindingFlags.NonPublic);

				// In the latest Normcore versions you need to access IncrementFixedRoomTime and RemoteFixedUpdate via the base type.
				// Replace the above two lines with the following three lines if you get errors about methods not being able to be found. 
				// I haven't tested these changes myself, but thanks to Erenimooo and Utku from the normcore discord for providing this fix.
				
				// var strategyBaseType = strategyValue.GetType().BaseType;
				// _incrementMethod = strategyBaseType.GetMethod("IncrementFixedRoomTime", BindingFlags.Instance | BindingFlags.NonPublic);
				// _remoteFixedUpdateMethod = strategyBaseType.GetMethod("RemoteFixedUpdate", BindingFlags.Instance | BindingFlags.NonPublic);

				_isStaticInitialized = true;
			} else {
				strategyValue = _strategyField.GetValue(_realtimeTransform);
			}

			_realtimeTransformModel = (RealtimeTransformModel) _modelProperty.GetValue(_realtimeTransform);
			_incrementFixedRoomTime = (Action) Delegate.CreateDelegate(typeof(Action), strategyValue, _incrementMethod, true);
			_remoteFixedUpdate = (Action<RealtimeTransformModel>) Delegate.CreateDelegate(typeof(Action<RealtimeTransformModel>), strategyValue, _remoteFixedUpdateMethod, true);

			_realtimeTransform.realtime.didConnectToRoom += HandleConnected;
			_realtimeTransform.realtime.didDisconnectFromRoom += HandleDisconnected;

			_clientId = _realtimeTransform.realtime.clientID;

			if (_realtimeTransform.ownerIDSelf == _clientId) {
				_isAlreadyOwned = true;
				_rigidbody.WakeUp();
			}

			_isInitialized = true;
		}
	}
}

Updated with a few improvements:

• Cache static references to some of the methods and properties accessed via reflection.
• Use FixedUpdate instead of a coroutine using WaitForFixedUpdate (note: this changes the order of execution but hasn't caused any issue in my tests https://docs.unity3d.com/Manual/ExecutionOrder.html)
• Handle disconnects
• Support null references for the realtime components to handle cases where realtime components are stripped at runtime.

This has been reliable for Realtime prefab instances in my project, but I haven't tested SceneView RealtimeTransforms with Rigidbodies.

If you're using the latest normcore versions, you'll need to apply the fix mentioned in the comments of HandleReplacedAllComponentModels.