Laggy MacBook Pro on High Sierra and 2 external displays I've got a late 2013, 15-inch retina MacBook Pro.
Since a couple macOS upgrades ago, my system has become randomly slow, borderline unusable. I've come to realize it spikes when using 3 displays (2 external 1080p and laptop's retina screen). If I close the laptop and only use the 2 external displays, CPU usage instantly drops. And then, if I open it again, rises up and kernel_task can report over 150% CPU usage and transitions, typing, everything becomes laggy. This can happen even on a fresh boot.
I'm clueless, since 3 displays should be effortless on the GPUs task and by no means take over the CPU. I read kernel_task might spike in order to prevent other processes to hog the CPU and overheat. Even then, closing the screen interrupts no CPU consuming processes, so that shouldn't calm kernel_task down. And the laptop should be even hotter while closed than open.
Is there a step that might help resolve the slowdown?
I don't know why I didn't try this earlier, but a SMC Reset did it. 

So I guess sensors were a little off and that was it? Any additional technical details is still much appreciated.