Choppy video playback

[...don]

I have assigned 4 of the 8 hyper-threads to the VM (50%) which is in the green region on the System settings tab. The Settings tab does call them CPUs, but indicates that there are 8 available. Typically none of the hyper-threads are running > 30% CPU.

I have reverted to VB 5.2.22 with the 6.0 RC1 extensions in the VM. With this combination the video playback is smooth, the video and audio are in sync, and there is no mouse pointer offset. There is no noticeable difference with or without 2d acceleration.

[socratis]

00:00:01.763330 CPUM: Logical host processors: 8 present, 8 max, 8 online, online mask: 00000000000000ff
00:00:01.763438 CPUM: Physical host cores: 4
00:00:01.763741 Full Name: "Intel(R) Core(TM) i7-6700HQ CPU @ 2.60GHz"

Your i7-6700HQ has 4 cores, 8 threads. In that Intel linked webpage if you look at the #Cores and #Threads, see how Intel describes them (click at the "?" part). VirtualBox cares about the hardware capabilities, not the software (marketing) ones.

I have reverted to VB 5.2.22 with the 6.0 RC1 extensions in the VM. With this combination the video playback is smooth

The Guest Additions, have nothing to do with the sound part. And BTW, it's Guest Additions, not extensions, you might get confused with the Extension Pack, which gets installed on the host.