Nobody ever said that tick would be regular.
Tick happens once per frame, if the system load in each frame changes, so does tick.
The load is very irregular as the system starts up.
If you want an accurate time, then you can use a timer node 
Nobody ever said that tick would be regular.
Tick happens once per frame, if the system load in each frame changes, so does tick.
The load is very irregular as the system starts up.
If you want an accurate time, then you can use a timer node