The task manager is showing small HDD usage not because it doesn’t need to utilize more but because of the limitation of the mechanical parts, it’s showing you a % of potential maximum sequential read/write. I recommend reading this article for more information: Should You Upgrade? From A Hard Drive To An SSD (it’s quite old, but the comparison is still valid, if anything the SSDs got much better and cheaper now).
As for the editor problem, looking at the log it seems you DO have an infinite loop, here:
[2016.03.08-01.51.53:101][609]LogScriptCore: Script call stack:
Function /Game/3DMazeBarebones.3DMazeBarebones_C:UserConstructionScript
Function /Game/3DMazeBarebones.3DMazeBarebones_C:PickNearbyTile
[2016.03.08-01.51.53:101][609]LogScript:Warning: Runaway loop detected (over 1000000 iterations)
3DMazeBarebones_C /Game/3DMaze.3DMaze:PersistentLevel.3DMazeBarebones_40
Function /Game/3DMazeBarebones.3DMazeBarebones_C:PickNearbyTile:01EE
So look in your PickNearbyTile loops, or if you can, paste the screenshots here.