Linux guest (Slackware) extremely slow after updating Guest Additions

[JD]

I have a guest running Slackware 14.2 under Windows 7, with VirtualBox 6.0.8. Everything was fine until I said, "Self, you haven't updated the Guest Additions to the latest version in a while", so I went and did that. After rebooting the guest, it is unusably slow. It boots normally until it gets up to

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[drm] Initialized vboxvideo 1.0.0 20130823 for 0000:00:02.0 on minor 0

After that the boot seems to hang for several minutes. It does eventually complete, I get a login prompt, and can log in, but the console takes about 20 seconds to respond to any keypress. I upgraded to 6.0.10 to make sure I was on the latest version with any bugfixes, but the behavior is unchanged. Thoughts/suggestions/places to start troubleshooting?

The host information is as follows:
Windows 7 Professional
Processor: Intel Core i5-6300U @ 2.4 GHz
Memory: 8 GB

The guest information is as follows:
Slackware 14.2
Base memory: 2048 MB
Processors: 2
Acceleration: VT-x/AMD-V, Nested Paging, PAE/NX, KVM Paravirtualization
Video Memory: 64 MB
Screens: 2
Graphics Controller: VBoxVGA

I have attached a log file.

[JD]

I just noticed something weird. I hadn't even tried starting X because the system was so slow, but once I did, it's perfectly fast. I can type at a normal speed, open programs, drag windows around, etc., all normally. Only switching between windowed/fullscreen/seamless is slow. So whatever it is appears to be limited to the console.