The Moodle task system is not designed to run to minute-accurate timing.
It works based on whether or not the due time for a task has been reached (or past), when it is reached in a queue.
So, if there are 2 slow tasks, then everything else could get held up by 2 minutes (plus any time it takes other tasks to run), 3 slow-running tasks could hold everything else up for 3 minutes.
In general, I don't see lots of slow-running tasks that take longer than 1 minute to run, but it is entirely dependent on the volume of data and what is happening on a site (e.g. a site that has 1000s of forum posts per hour, will take a lot longer to send out the scheduled forum post emails, than a site which uses very few forums; if email notifications are turned off for all forum posts, then that task will take no time at all ...).
If you have some tasks where the exact timing is mission-critical, then you may have to look at alternative ways to run those task (direct calling of the scheduled task, running them external to the Moodle system?)