I thought I would post a thread clarification, as it seems it's being diluted with multiple issues which may not be related to the original post. As I understand it, the original post reported lagging, where everything would freeze, except perhaps the pointer, for 5 to 15 seconds, and then return to normal operation. The OP also mentions a seeming hard freeze, where everything save the pointer would freeze, including the clock seconds, and it would never unfreeze, necessitating a forced shutdown/reboot.
What seems to be confusing some people is the mention that, if the machine hard freezes with no recovery, sometimes the sound on a video or from iTunes would continue playing in the background. In my case, a really hefty file-copy operation continued and even completed in the background after one hard freeze.
These symptoms seem to happen only on iMacs and MacBook Pros, across multiple configurations and year models, including all video cards, SSD/HDs and RAM configs, and in all Web browsers.
As a personal update, I've not had the issue recur since installing the 10.13.2 Combo Update yesterday. I did see that 10.13.2 includes updated video drivers, which would confirm several people's speculation that the issue centers around the new Metal 2 implementation, since it's being leaned on much more heavily in the OS, for window drawing in the Finder and Safari, for example. I will post an update once I've used the updated system more.
