Good implementation of POST-Redirect-GET in ASP.NET Core MVC preserving user input and validation messages considering performance

Question

I have a multi-page form to submit and update an application for an event built using ASP.NET Core MVC. I want it to use the POST-Redirect-GET pattern, but without losing the input fields contents and the validation errors. Each page of the form consists of the display information managed by the organizer (1) and the information entered by the user (2) (e.g. a list of locations of venues (1) the the choice of venues (2)). I have a working implementation, but I'm not sure about performance. I can best explain my approach using (C#) code:

using System.Text.Json;
using Microsoft.AspNetCore.Mvc;

public class ApplicationController : Controller
{
    [HttpGet]
    public async Task<IActionResult> Step1(int id)
    {
        this.TempData.TryGetValue("viewModel", out Object? viewModelJSON);
        Step1ViewModel viewModel;
        if (viewModelJSON == null)
        {
           // create empty view model
           viewModel = new Step1ViewModel();
           viewModel.ID = id;
           // when simple GET: show data from db
           var userInputModel = await this.LoadVenueChoicesAsync(id);
           // fill view model if data exists
           if (userInputModel != null) {
              viewModel.UserInput = BuildUserInputViewModel(userInputModel);
           }
        }
        else
        {
            // when GET after POST: show input
            viewModel = JsonSerializer.Deserialize<Step1ViewModel>(viewModelJSON);
            await this.TryValidateModelAsync(viewModel);
        }
        var venueList = await this.LoadVenuesAsync();
        viewModel.Information = BuildInformationViewModel(venueList);

        return this.View(viewModel);
    }

    [HttpPost]
    public async Task<IActionResult> Step1(Step1ViewModel viewModel)
    {
        if (this.ModelState.IsValid == false)
        {
            var viewModelJSON = JsonSerializer.Serialize<Step1ViewModel>(viewModel);
            this.TempData["viewModel"] = viewModelJSON;
            return this.RedirectToAction("Step1", new { id = viewModel.ID });
        }
        else
        {
            await this.StoreVenueChoicesAsync(viewModel.UserInput);
            return this.RedirectToAction("Step2", new { id = viewModel.ID });
        }
    }
}

The web form is going to be used for a short time by many users, so performance must be good. Because of that I would like to validate this approach. Some specific questions I have:

• Will the serialization to JSON be a performance issue? Should I use something else?
• The learn.microsoft.com page, states Session should be used with LoadAsync() method to avoid performance issues. Because TempData uses Session, I wonder whether I should call this method as well?
• Are there other issues with this approach? Thank you for all input.