I use VirtualBox for testing in many environments. Normally the defaults are fine for installing Linux but on one environment it was really slow to just install. Installing Debian Jessie from an ISO would normally take 15 minutes but in this environment after an hour it was still only at 35%. The host OS was not over taxed and has lots of spare CPU + RAM resources free.
After some testing I found the issue was resolved by :
1. Open Settings
2. Navigate to System->Motherboard
3. Change the Chipset to ICH9 (see below)
4. Press OK
Now the installation will be fast.