mutex-actor.swift

import Synchronization
import SwiftUI

final class Counter1: Sendable {
  let count = Mutex(0)
  func increment() {
    count.withLock { $0 += 1 }
  }
  func increment10000() {
    for _ in 1...10_000 {
      increment()
    }
  }
}
final actor Counter2 {
  var count = 0
  func increment() {
    count += 1
  }
  func increment10000() {
    for _ in 1...10_000 {
      increment()
    }
  }
}

struct ContentView: View {
  var body: some View {
    Form {
      Button("Mutex") {
        let start = Date()
        defer { print("mutex", Date().timeIntervalSince(start).formatted()) }
        let counter = Counter1()
        counter.increment10000()
      }
      Button("Actor") {
        Task {
          let start = Date()
          defer { print("actor", Date().timeIntervalSince(start).formatted()) }
          let counter = Counter2()
          await counter.increment10000()
        }
      }
    }
  }
}

Yes it generally is true, and it only becomes more true as the logic in the lock becomes more complex.