This is a strange one, and I'm trying to figure it out, but I'm coming up somewhat empty-handed.

A new El Capitan VM built using the Packer template for VirtualBox will freeze for ~5 seconds every ~10 seconds, making interactive use somewhat troublesome. However, VirtualBox's OS X support is not necessarily broken outright.

Here's what I've observed:

• If I build a straight DMG using these steps and VirtualBox's default OS X 10.11 configuration, the resulting VM does not freeze at all.
• If I modify the Packer template to build a machine that more closely resembles VirtualBox's default configuration, the resulting box still freezes. (I've diffed the .vbox files—they're pretty much identical save UUIDs and names of things.)
• The VM does not appear to freeze during the initial installation from the installation media. You can see the freezing if you swirl your mouse around inside the VM's console window for >10 seconds.
• I noticed that shrink.sh is failing due to System Integrity Protection not allowing dynamic_pager to be unloaded, but if I attempt to build a box with these lines commented out, the resulting box still freezes.

It seems that something installed in the prepare_iso process is responsible, since the freezing is in play even before Packer starts running the scripts in scripts. But I've gone over what prepare_iso adds and I'm not sure what could be causing it.

Has anyone seen this or does anyone have any ideas what might be at fault? I'm currently looking over pkg-postinstall to see if anything looks suspicious.