When you account for not knowing what else is going on the system I'd say it's actually non deterministic. But not in a magical "truly random" sort of way, just that other things you don't personally have control over are going on. If this topic interests you then you may want to look into real time computing which is an area where you do have deterministic systems where you can more accurately guarantee how long something will take. This is important in dangerous activities. Think things like nuclear reactors where a process taking too long might mean not alerting another part of a system that something bad has happened. Like the part of the system that tells you if something is too hot not responding so you keep adding fuel. Compare this to your phone. If your phone is slow then, well, it's just annoying really.