virtualbox.org

I've had a VM that I've been using to test a build. I reboot it to an ISO image which installs a base OS and go from there. That's worked for months. I used it under Mountain Lion, then Mavericks, then Yosemite. I was up to 4.3.18 Yesterday, it became really slow. I'd type commands, and the terminal would sit there and then echo the letters a few seconds later. I saw 4.3.20 was out, so I installed it. No change. Rebooted my system. No change. Tried in another VM, and it worked. Deleted and recreated my VM, and it's slow.

I'm having a hard time imagining what the heck might be causing this

Host is 8GB, VM is Red Hat 64 bit 1GB

What is the VM?
See Minimum information needed for assistance.