ViewController.swift

import UIKit

class ViewModel {
    func heavyCall() {
        var count = 0
        while count < 1_000_000_000 {
            count += 1
        }
        print(Thread.isMainThread)  // false
    }
}

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()

        Task.detached {
            let viewModel = ViewModel()
            viewModel.heavyCall()
        }
    }
}