Okay, I remembered that good old Kadira has a CPU profiling feature based on v8-profiler on my paid plan.
Cool thing is I can share them, so here is the CPU profile I took:
Except for the client requesting the CPU profile, there were 0 other connected clients. So the application should be idle, but was running at 100% CPU.
I'm yet to learn how to interpret the profile correctly, so if anyone is experienced in this, he might come to a conclusion faster...