This isn't a bug, it just means you are usually limited by your CPU.
Since your CPU can't compute fast enough, your GPU can never get fully utilized in normal games, in sandbox however, there is much less to compute so your GPU can just go nuts. High GPU usage is good, it means you're getting...
The side exits to spawn facing the outer two lanes by the two inner guardians can be bypassed by using warpstone/wraith's teleport. It is possible to then walk out from the middle upper door or shoot the enemies inside their own spawn without taking damage.
Here is a video showing this in...
When you get the notification in the top-right "This match is now safe to abandon without penalty.", there is no reason to keep fighting, especially if it's early in the game. Playing a 5v6 game doesn't really make sense as a time commitment, so people just leave.
Same issues, 6700XT, 12100f. Getting 140+ usually but it dips to 30-40 FPS in teamfights, the issue is also present on my friend's and team's computers.