Shard 2 was experiencing some issues this afternoon taking well over 10s to execute each tick. During this time I noticed that a few of the creeps my neighbor has been consistently sending at me were not dying in the same space as usual, so I peeked back through my room history to see how he had snuck around my defenses.
As it turns out, I had no defenses. In fact, I had no code executing AT ALL. My CPU usage was at 0, my console was devoid of errors, and enemy creeps were still moving around in my room.
I decided to switch back to Shared-VM from the new Isolated-VM runtime on a whim, and BAM, my room was up and running again.
A particularly bad stutter: https://screeps.com/a/#!/history/shard2/E8S14?t=4056180
When I changed back to Shared-VM: https://screeps.com/a/#!/history/shard2/E8S14?t=4056350