I just upgraded from a 6700k to a 9900k because the 6700k was just not keeping up in 8.1. I'm running a GTX 980 4Gb vs. your GTX 1060 and all of my system related lag, delays, stuttering, freezes, etc. were alleviated with the processor upgrade. The reason your 2 & 5 slot toons always take longer is that they are essentially sharing the same CPU resources. It was the same with my setup and using the default CPU setup of dedicating 1 CPU to slot 1 and sharing the other 3 between the other 4 clients.

Now it's entirely possible that the lock ups are unrelated but the CPU would be my first guess. Previously if I switched my settings from DX 12 to DX 11 it would hang all of my WOW sessions and I'd have to Alt+F4 out and re-launch, though the setting change took effect. Now I can make the same change with no negative impact and all clients accept the changed settings without issue.

Just my $0.02 based on my recent experience