I have had a 1st level party take out a owlbear before the owlbear even had a chance to go. I have had a 5th party take out a 13CR big bad when he was placed there to scare them away and foreshadow the end.
Granted my guys are on the edge of powergaming at times and very experienced but yeah if they are fresh and feel free to nova......little in the game threatens them unless I go into ridiculous CR and numbers for their levels.
Now this group rolls stats. Uses feats and goes out of their way to hunt down magic items as well as works very well as a team.
I can wear them down over time but even that is sometimes impossible to do at the mid/higher levels with so many easy ways to grab a short or even long rest with spells.