[SOLVED] LXC with more cores assigned uses dramatically less CPU. Why?

no, this is "Load" which is an absolute number (of processes running or waiting to be executed, or rather, the average of that number over certain time windows). obviously it scales with the number of CPUs (more CPUs == more stuff can run), but it's not limited by the number of CPUs (in fact, having load > # of cores indicates a system that is using its CPUs, or might benefit from having more CPUs). in general it's a rather poor metric, which is why the new PSI (pressure stall information) values were added in the kernel.
 
I know the matter has been resolved, but just for reference, here's what I was referring to:

~# uptime
14:17:19 up 21 days, 19:44, 1 user, load average: 5.37, 5.36, 5.55

This refers to CPU's, not percentages.
No it does not. This refers to the active entries in the run queue and it is NOT the cpu usage. You can have active entries and no CPU usage at all. Two weeks ago, we had as server with a load over 8000:

Code:
root@xxx ~ > w
 11:56:19 up 139 days,  2:11,  4 users,  load average: 8244.56, 8243.99, 8242.80

CPUs were only 1/8 used (that is 20%).