To reproduce:

1. Download attached project "request_tests.zip" and open in Unity
2. Open "requests" scene
3. Set Scripting Runtime Version to .NET 4.x
4. Set Scripting Backend to IL2CPP
5. Build for iOS or Android
6. Deploy the built project to the low-end device
7. Press "Do Setup" and then press "Start" beside "Best HTTP"

Expected behavior: The time counter does not stutter
Actual behavior: The time counter stutter

Notes:
- This issue does not reproduce on Windows Standalone and macOS Standalone
- This issue appears only on iOS and Android (IL2CPP)
- If you use .NET 3.5 or Mono the stuttering doesn't occur
- If you turn off SSL Stream the stuttering doesn't occur
- If you press the "Start" button beside "Unity" it will make the same requests but through UnityWebRequest - this doesn't stutter either

Tested with:
- iPod Touch 5th Gen, iOS: 9.3.5 - Reproduced
- iPhone 5, iOS: 10.1.1 - Reproduced
- Lge Nexus 5 (Nexus 5), Android 6.0.1, CPU: Snapdragon 800 MSM8974, GPU: Adreno (TM) 330 - Reproduced
- iPhone SE, iOS: 11.4.1 - Not Reproduced
- iPhone 7s Plus, iOS: 12.0 - Not Reproduced

Reproduced on Unity 2018.1.0b1, 2018.1.9f2, 2018.2.18f1, 2018.3.0b12 and 2019.1.0a10
Not reproduced on Unity 2017.4.16f1 and 2018.1.0a7
Regression on Unity 2018.1.0b1