To reproduce:
1. Create a new C# script and paste in:

using UnityEngine;
using UnityEditor;
using System.Collections;
[InitializeOnLoad]
class Test {
static Test() {
EditorApplication.update += OnUpdate;
}
private static int s_Counter = 0;
//private static float s_Tim = 0;
private static void OnUpdate() {
// s_Tim += Time.deltaTime;
if (++s_Counter == 200) {
s_Counter = 0;
Debug.Log("Tick");
//s_Tim = 0;
}
}
}

It prints out a tick after every 200 updates. After first looking into the console you should notice that it seems quite fast - a lot faster than the stated 100 updates per second in the documentation.

2. Uncomment the comments in the script and add + s_Tim.toString() to the debug log. The updates slow down
3. Enter play mode - you should notice that the time value is always the same (for me it was 4 seconds, which is half the speed in the documentation)
4. Comment the time counting parts again - the slow speed still remains

Basically there are some weird inconsistencies, most likely they are by design, but more accurate notes in the documentation would be nice